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Darling, t[1].   well logging and formation evaluation (2005)
 

Darling, t[1]. well logging and formation evaluation (2005)

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    Darling, t[1].   well logging and formation evaluation (2005) Darling, t[1]. well logging and formation evaluation (2005) Document Transcript

    • WELL LOGGING AND FORMATION EVALUATION
    • WELL LOGGING AND FORMATION EVALUATION Toby Darling AMSTERDAM • BOSTON • HEIDELBERG • LONDON NEW YORK • OXFORD • PARIS • SAN DIEGO SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO Gulf Professional Publishing is an imprint of Elsevier Science
    • Gulf Professional Publishing is an imprint of Elsevier 30 Corporate Drive, Suite 400, Burlington, MA 01803, USA Linacre House, Jordan Hill, Oxford OX2 8DP, UK Copyright © 2005, Elsevier Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher. Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone: (+44) 1865 843830, fax: (+44) 1865 853333, e-mail: permissions@elsevier.com.uk. You may also complete your request on-line via the Elsevier homepage (http://elsevier.com), by selecting “Customer Support” and then “Obtaining Permissions.” Recognizing the importance of preserving what has been written, Elsevier prints its books on acid-free paper whenever possible. Library of Congress Cataloging-in-Publication Data Application submitted. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. ISBN: 0-7506-7883-6 For information on all Gulf Professional Publishing publications visit our Web site at www.books.elsevier.com 05 06 07 08 09 10 10 9 8 7 6 5 4 3 2 1 Printed in the United States of America Working together to grow libraries in developing countries www.elsevier.com | www.bookaid.org | www.sabre.org
    • CONTENTS Introduction ix 1 Basics 1 1.1 Terminology 1 1.2 Basic Log Types 3 1.3 Logging Contracts 9 1.4 Preparing a Logging Programme 1.5 Operational Decisions 14 1.6 Coring 16 1.7 Wellsite Mud Logging 21 1.8 Testing/Production Issues 24 11 2 Quicklook Log Interpretation 29 2.1 Basic Quality Control 29 2.2 Identifying the Reservoir 30 2.3 Identifying the Fluid Type and Contacts 32 2.4 Calculating the Porosity 34 2.5 Calculating Hydrocarbon Saturation 37 2.6 Presenting the Results 40 2.7 Pressure/Sampling 42 2.8 Permeability Determination 45 3 Full Interpretation 49 3.1 Net Sand Definition 49 3.2 Porosity Calculation 51 3.3 Archie Saturation 53 3.4 Permeability 54 4 Saturation/Height Analysis 59 4.1 Core Capillary Pressure Analysis 4.2 Log-Derived Functions 64 v 60
    • vi Contents 5 Advanced Log Interpretation Techniques 67 5.1 Shaly Sand Analysis 67 5.2 Carbonates 73 5.3 Multi-Mineral/Statistical Models 74 5.4 NMR Logging 76 5.5 Fuzzy Logic 85 5.6 Thin Beds 87 5.7 Thermal Decay Neutron Interpretation 93 5.8 Error Analyses 96 5.9 Borehole Corrections 101 6 Integration with Seismic 103 6.1 Synthetic Seismograms 103 6.2 Fluid Replacement Modelling 108 6.3 Acoustic/Elastic Impedance Modelling 110 7 Rock Mechanics Issues 115 8 Value Of Information 9 Equity Determinations 125 9.1 Basis for Equity Determination 126 9.2 Procedures/Timing for Equity Determination 9.3 The Role of the Petrophysicist 129 119 10 Production Geology Issues 137 10.1 Understanding Geological Maps 140 10.2 Basic Geological Concepts 147 11 Reservoir Engineering Issues 155 11.1 Behavior of Gases 155 11.2 Behavior of Oil/Wet Gas Reservoirs 11.3 Material Balance 162 11.4 Darcy’s Law 163 11.5 Well Testing 166 159 12 Homing-in Techniques 171 12.1 Magnetostatic Homing-in 171 12.2 Electromagnetic Homing-in 185 13 Well Deviation, Surveying, and Geosteering 13.1 Well Deviation 193 13.2 Surveying 195 193 127
    • vii Contents 13.3 13.4 13.5 Geosteering 197 Horizontal Wells Drilled above a Contact 203 Estimating the Productivity Index for Long Horizontal Wells 205 Appendix 1 Test Well 1 Data Sheet 207 Appendix 2 Additional Data for Full Evaluation Appendix 3 Solutions to Exercises 215 218 Appendix 4 Additional Mathematics Theory Appendix 5 Abbreviations and Acronyms 251 264 Appendix 6 Useful Conversion Units and Constants Appendix 7 Contractor Tool Mnemonics Bibliography 309 About the Author 313 Acknowledgments 314 Index 315 271 268
    • INTRODUCTION The purpose of this book is to provide a series of techniques which will be of real practical value to petrophysicists in their day-to-day jobs. These are based on my experience from many years working in oil companies. To this end I have concentrated wherever possible on providing one recommended technique, rather than offer the reader a choice of different options. The primary functions of a petrophysicist are to ensure that the right operational decisions are made during the course of drilling and testing a well—from data gathering, completion and testing—and thereafter to provide the necessary parameters to enable an accurate static and dynamic model of the reservoir to be constructed. Lying somewhere between Operations, Production Geology, Seismology, Production Technology and Reservoir Engineering, the petrophysicist has a key role in ensuring the success of a well, and the characterization of a reservoir. The target audience for this book are operational petrophysicists in their first few years within the discipline. It is expected that they have some knowledge of petroleum engineering and basic petrophysics, but lack experience in operational petrophysics and advanced logging techniques. The book also may be useful for those in sister disciplines (particularly production geology and reservoir engineering) who are using the interpretations supplied by petrophysicists. ix
    • C H A P T E R 1 BASICS 1.1 TERMINOLOGY Like most professions, petroleum engineering is beset with jargon. Therefore, it will make things simpler if I first go through some of the basic terms that will be used throughout this book. Petroleum engineering is principally concerned with building static and dynamic models of oil and gas reservoirs. Static models are concerned with characterizing and quantifying the structure prior to any production from the field. Hence, key parameters that the models aim to determine are: • • • • • • • • • • STOIIP = stock tank oil initially in place; usually measured in stock tank barrels (stb) GIIP = gas initially in place; usually measured in billion standard cubic feet (Bcf) GBV = gross bulk volume; the total rock volume of the reservoir containing hydrocarbon NPV = net pore volume; the porespace of the reservoir HCPV = hydrocarbon pore volume; the porespace actually containing hydrocarbon f = porosity; the proportion of the formation that contains fluids k = permeability; usually expressed in millidarcies (md) Sw = water saturation; the proportion of the porosity that contains water Sh = hydrocarbon saturation; the proportion of the porosity that contains hydrocarbon FWL = free water level; the depth at which the capillary pressure in the reservoir is zero; effectively the depth below which no producible hydrocarbons will be found 1
    • 2 • • • • • Well Logging and Formation Evaluation HWC = hydrocarbon/water contact; the depth below which the formation is water bearing as encountered in a particular well. Likewise, OWC for oil and GWC for gas GOC = gas oil contact; the depth below which any gas in the reservoir will be dissolved in the oil Gross thickness = the total thickness of the formation as encountered in a particular well Net thickness = the part of the gross thickness that contains porous rock subject to given cutoff criteria Pay thickness = the part of the net thickness that is considered to be capable of producing hydrocarbons in a particular well Because of inherent uncertainties in all the parameters used to determine STOIIP or GIIP, geologists will usually develop probabilistic models, in which all the parameters are allowed to vary according to distribution functions between low, expected, and high values. The resulting static models may then be analyzed statistically to generate the following values, which are used for subsequent economic analyses: • • • • P50 STOIIP: the value of the STOIIP for which there is a 50% chance that the true value lies either above or below the value P15 STOIIP: the value of the STOIIP for which there is only a 15% chance that the true value exceeds the value. Often called the high case. P85 STOIIP: the value of the STOIIP for which there is an 85% chance that the true value exceeds the value. Often called the low case. Expected STOIIP: the value of the STOIIP derived by taking the integral of the probability density function for the STOIIP times the STOIIP. For a symmetric distribution, this will equal the P50 value. Similar terminology applies to GIIP. In order to predict the hydrocarbons that may be actually produced from a field (the reserves), it is necessary to construct a dynamic model of the field. This will generate production profiles for individual wells, subject to various production scenarios. Additional terminology that comes into play includes: • Reserves = the part of the STOIIP or GIIP that may be actually produced for a given development scenario. Oil companies have their own rules for how reserves are categorized depending on the extent to which they are regarded as proven and accessible through wells. Terms fre-
    • Basics • • • • • • 3 quently used are proven reserves, developed reserves, scope for recovery reserves, probable reserves, and possible reserves. Remaining reserves = that part of the reserves that has not yet been produced Cumulative production = that part of the reserves that has already been produced UR = ultimate recovery; the total volume of reserves that will be produced prior to abandonment of the field NPV = net present value; the future economic value of the field, taking into account all future present value costs and revenues RF = recovery factor; the reserves as a proportion of the STOIIP (or GIIP) Bo = oil volume factor; the factor used to convert reservoir volumes of oil to surface (stock tank) conditions. Likewise Bg for gas. In order to produce the hydrocarbons, wells are needed and a development strategy needs to be constructed. This strategy will typically be presented in a document called the field development plan (FDP), which contains a summary of current knowledge about the field and the plans for future development. Once an FDP has been approved, the drilling campaign will consist of well proposals, in which the costs, well trajectory, geological prognosis, and data-gathering requirements are specified. The petrophysicist plays a part in the preparation of the well proposal in specifying which logs need to be acquired in the various hole sections. 1.2 BASIC LOG TYPES Below is a list of the main types of logs that may be run, and why they are run. 1.2.1 Logging While Drilling (LWD) Traditionally, petrophysicists were concerned only with wireline logging, that is, the data acquired by running tools on a cable from a winch after the hole had been drilled. However, advances in drilling/logging technology have allowed the acquisition of log data via tools placed in the actual drilling assembly. These tools may transmit data to the surface on a real-time basis or store the data in a downhole memory from which it may be downloaded when the assembly is brought back to the surface.
    • 4 Well Logging and Formation Evaluation LWD tools present a complication for drilling, as well as additional expense. However, their use may be justified when: • • • • Real-time information is required for operational reasons, such as steering a well (e.g., a horizontal trajectory) in a particular formation or picking of formation tops, coring points, and/or casing setting depths Acquiring data prior to the hole washing out or invasion occurring Safeguarding information if there is a risk of losing the hole The trajectory is such as to make wireline acquisition difficult (e.g., in horizontal wells) LWD data may be stored downhole in the tools memory and retrieved when the tool is brought to the surface and/or transmitted as pulses in the mud column in real time while drilling. In a typical operation, both modes will be used, with the memory data superseding the pulsed data once the tool is retrieved. However, factors that might limit the ability to fully use both sets of data are: • • • • Drilling mode: Data may be pulsed only if the drillstring is having mud pumped through it. Battery life: Depending on the tools in the string, tools may work in memory mode only between 40 and 90 hours. Memory size: Most LWD tools have a memory size limited to a few megabytes. Once the memory is full, the data will start to be overwritten. Depending on how many parameters are being recorded, the memory may become full within 20–120 hours. Tool failure: It is not uncommon for a fault to develop in the tool such that the pulse data and/or memory data are not transmissible/ recordable. Some of the data recorded may be usable only if the toolstring is rotating while drilling, which may not always be the case if a steerable mud motor is being used. In these situations, the petrophysicist may need to request drilling to reacquire data over particular intervals while in reaming/rotating mode. This may also be required if the rate of penetration (ROP) has been so high as to affect the accuracy of statistically based tools (e.g., density/neutron) or the sampling interval for tools working on a fixed time sampling increment. Another important consideration with LWD tools is how close to the bit they may be placed in the drilling string. While the petrophysicist will obviously want the tools as close to the bit as possible, there may be
    • Basics 5 limitations placed by drilling, whose ability to steer the well and achieve a high ROP is influenced by the placement of the LWD toolstring. LWD data that may typically be acquired include the following: • • • • • GR: natural gamma ray emission from the formation Density: formation density as measured by gamma ray Compton scattering via a radioactive source and gamma ray detectors. This may also include a photoelectric effect (Pe) measurement. Neutron porosity: formation porosity derived from the hydrogen index (HI) as measured by the gamma rays emitted when injected thermal or epithermal neutrons from a source in the string are captured in the formation Sonic: the transit time of compressional sound waves in the formation Resistivity: the formation resistivity for multiple depths of investigation as measured by an induction-type wave resistivity tool Some contractors offer LWD-GR, -density, and -neutron as separate up/down or left/right curves, separating the contributions from different quadrants in the borehole. These data may be extremely useful in steering horizontal wells, where it is important to determine the proximity of neighboring formation boundaries before they are actually penetrated. Resistivity data may also be processed to produce a borehole resistivity image, useful for establishing the stratigraphic or sedimentary dip and/or presence of fractures/vugs. Other types of tool that are currently in development for LWD mode include nuclear magnetic resonance (NMR), formation pressure, and shear sonic. 1.2.2 Wireline Openhole Logging Once a section of hole has been completed, the bit is pulled out of the hole and there is an opportunity to acquire further openhole logs either via wireline or on the drillstring before the hole is either cased or abandoned. Wireline versions of the LWD tools described above are available, and the following additional tools may be run: • • Gamma ray: This tool measures the strength of the natural radioactivity present in the formation. It is particularly useful in distinguishing sands from shales in siliciclastic environments. Natural gamma ray spectroscopy: This tool works on the same principal as the gamma ray, although it separates the gamma ray counts into
    • 6 • • • • • • • • Well Logging and Formation Evaluation three energy windows to determine the relative contributions arising from (1) uranium, (2) potassium, and (3) thorium in the formation. As described later in the book, these data may be used to determine the relative proportions of certain minerals in the formation. Spontaneous potential (SP): This tool measures the potential difference naturally occurring when mud filtrate of a certain salinity invades the formation containing water of a different salinity. It may be used to estimate the extent of invasion and in some cases the formation water salinity. Caliper: This tool measures the geometry of the hole using either two or four arms. It returns the diameter seen by the tool over either the major or both the major and minor axes. Density: The wireline version of this tool will typically have a much stronger source than its LWD counterpart and also include a Pe curve, useful in complex lithology evaluation. Neutron porosity: The “standard” neutron most commonly run is a thermal neutron device. However, newer-generation devices often use epithermal neutrons (having the advantage of less salinity dependence) and rely on minitron-type neutron generators rather than chemical sources. Full-waveform sonic: In addition to the basic compressional velocity (Vp) of the formation, advanced tools may measure the shear velocity, Stonely velocity, and various other sound modes in the borehole, borehole/formation interface, and formation. Resistivity: These tools fall into two main categories: laterolog and induction type. Laterolog tools use low-frequency currents (hence requiring water-based mud [WBM]) to measure the potential caused by a current source over an array of detectors. Induction-type tools use primary coils to induce eddy currents in the formation and then a secondary array of coils to measure the magnetic fields caused by these currents. Since they operate at high frequencies, they can be used in oil-based mud (OBM) systems. Tools are designed to see a range of depths of investigation into the formation. The shallower readings have a better vertical resolution than the deep readings. Microresistivity: These tools are designed to measure the formation resistivity in the invaded zone close to the borehole wall. They operate using low-frequency current, so are not suitable for OBM. They are used to estimate the invaded-zone saturation and to pick up bedding features too small to be resolved by the deeper reading tools. Imaging tools: These work either on an acoustic or a resistivity principle and are designed to provide an image of the borehole wall that may
    • Basics • • • • • 7 be used for establishing the stratigraphic or sedimentary dip and/or presence of fractures/vugs. Formation pressure/sampling: Unlike the above tools, which all “log” an interval of the formation, formation-testing tools are designed to measure the formation pressure and/or acquire formation samples at a discrete point in the formation. When in probe mode, such tools press a probe through the mudcake and into the wall of the formation. By opening chambers in the tool and analyzing the fluids and pressures while the chambers are filled, it is possible to determine the true pressure of the formation (as distinct from the mud pressure). If only pressures are required (pretest mode), the chambers are small and the samples are not retained. For formation sampling, larger chambers are used (typically 23/4 or 6 gallons), and the chambers are sealed for analysis at the surface. For some tools, a packer arrangement is used to enable testing of a discrete interval of the formation (as opposed to a probe measurement), and various additional modules are available to make measurements of the fluid being sampled downhole. Sidewall sampling: This is an explosive-type device that shoots a sampling bullet into the borehole wall, which may be retrieved by a cable linking the gun with the bullet. Typically this tool, consisting of up to 52 shots per gun, is run to acquire samples for geological analysis. Sidewall coring: This is an advanced version of the sidewall sampling tool. Instead of firing a bullet into the formation, an assembly is used to drill a sample from the borehole wall, thereby helping to preserve the rock structure for future geological or petrophysical analyses. NMR: These tools measure the T1 and T2 relaxation times of the formation. Their principles and applicability are described in Chapter 5. Vertical seismic profiling (VSP): This tool fires a seismic source at the surface and measures the sound arrivals in the borehole at certain depths using either a hydrophone or anchored three-axis geophone. The data may be used to build a localized high-resolution seismic picture around the borehole. If only the first arrivals are measured, the survey is typically called a well shoot test (WST) or checkshot survey. VSPs or WSTs may also be performed in cased hole. 1.2.3 Wireline Cased Hole Logging When a hole has been cased and a completion string run to produce the well, certain additional types of logging tools may be used for monitoring purposes. These include:
    • 8 • • • • • Well Logging and Formation Evaluation Thermal decay tool (TDT): This neutron tool works on the same principle as the neutron porosity tool, that is, measuring gamma ray counts when thermal neutrons are captured by the formation. However, instead of measuring the HI, they are specifically designed to measure the neutron capture cross-section, which principally depends on the amount of chlorine present as formation brine. Therefore, if the formation water salinity is accurately known, together with the porosity, Sw may be determined. The tool is particularly useful when run in time-lapse mode to monitor changes in saturation, since many unknowns arising from the borehole and formation properties may be eliminated. Gamma ray spectroscopy tool (GST): This tool works on the same principal as the density tool, except that by measuring the contributions arising in various energy windows of the gamma rays arriving at the detectors, the relative proportions of various elements may be determined. In particular, by measuring the relative amounts of carbon and oxygen a (salinity independent), measurement of Sw may be made. Production logging: This tool, which operates using a spinner, does not measure any properties of the formation but is capable of determining the flow contributions from various intervals in the formation. Cement bond log: This tool is run to evaluate the quality of the cement bond between the casing and the formation. It may also be run in a circumferential mode, where the quality around the borehole is imaged. The quality of the cement bond may affect the quality of other production logging tools, such as TDT or GST. Casing collar locator (CCL): This tool is run in order to identify the positions of casing collars and perforated intervals in a well. It produces a trace that gives a “pip” where changes occur in the thickness of the steel. 1.2.4 Pipe-Conveyed Logging Where the borehole deviation or dogleg severity is such that it is not possible to run tools using conventional wireline techniques, tools are typically run on drillpipe. In essence, this is no different from conventional logging. However, there are a number of important considerations. Because of the need to provide electrical contact with the toolstring, the normal procedure is to run the toolstring in the hole to a certain depth before pumping down a special connector (called a wet-connect) to connect the cable to the tools. Then a side-entry sub (SES) is installed in the drillpipe, which allows the cable to pass from the inside of the pipe
    • Basics 9 to the annulus. The toolstring is then run in farther to the deepest logging point, and logging commences. The reason the SES is not installed when the toolstring is at the surface is partly to save time while running in (and allowing rotation), and also to avoid the wireline extending beyond the last casing shoe in the annulus. If the openhole section is longer than the cased hole section, the logging will need to be performed in more than one stage, with the SES being retrieved and repositioned in the string. Pipe-conveyed logging is expensive in terms of rig time and is typically used nowadays only where it is not possible to acquire the data via LWD. Most contractors now offer a means to convert an operation to pipeconveyed logging if a toolstring, run into the hole on conventional wireline, becomes stuck in the hole. This is usually termed “logging while fishing.” 1.3 LOGGING CONTRACTS Typically, an oil company will set up logging contracts with one or more contractors for the provision of logging services. Usually some kind of tendering process is used to ensure competitive bidding among various companies able to provide such services. Elements that exist in common contracts include the following: • • • • • • • • Depth charge: This relates to the deepest depth that a particular tool will be run in the hole. Survey charge: This relates to the interval that a particular tool is actually logged in the hole. Station charge: For tools such as formation pressure sampling tools and sidewall samples, this is a charge per station measurement. Usually the contract will make certain specifications regarding when such a charge may be dropped (e.g., if no useful data are recovered). Tool rental: Usually a daily charge for the tools to be on the rig on standby prior to or during a logging job Logging unit rental charge: Usually a monthly charge for the logging unit (winch, tool shed, and computers) while it is on the rig Base rental: There may be a monthly charge to have a pool of tools available for a client. For LWD tools, this may supersede the tool rental, depth, and survey charges. Engineer charge: Usually a day rate for any engineers, specialists, or assistants present for the logging job In-hole charge: Some LWD contracts specify an hourly charge while tools are actually being run in the hole.
    • 10 • • • • • Well Logging and Formation Evaluation Lost-in-hole charge: For replacement of any tools lost in the hole during operations. Some contractors provide insurance to the oil companies for a fixed sum per job to indemnify them against lost-in-hole charges. Cable splice charge: Where tools become stuck in the hole and it is necessary to cut the cable, a charge is usually made for such splicing. Processing charges: Where data require postprocessing (e.g., interpretation of image data or waveform sonic), charges are usually applied in a similar way to survey charges. Data charges: Provision of additional copies of log prints and/or tapes, or data storage, may incur additional charges. Real-time data transmission charges: The oil company will usually be given the option to have data transmitted directly from the wellsite to their office, either as digital data in Log ASCII Standard or binary format or as a print image. Most contracts offer the oil company a discount on the total monthly charges based on total volume of services called out during a particular month. Some oil companies operate incentive schemes that penalize the contractor financially based on lost time resulting from tool failures. There may also be bonuses based on good safety performance. When new tools being introduced by the contractor and not covered by the contract are proposed to be run, there will normally be some negotiation on special pricing. The oil company takes into account that if there is a testing element in the new tool being run, there is a benefit accruing to the contractor. Hence, the oil company may argue that the tool should be run free of charge initially. The contractor will usually argue that the oil company is benefiting from the tool’s technological advantages over alternative older-generation tools. Often a compromise is reached whereby the tool is run at a preferential pricing scheme (maybe equivalent to the price of the tool being replaced) for the first few runs until its usefulness has been proved. Typically the contractor will request the right to use the data acquired for future promotion of the tool, subject to confidentiality restrictions. Most oil companies will also specify, either in the contract or in a separate document, how data are to be delivered to their offices and what quality-control procedures should be followed during logging. Items typically specified in such a document will include: • • Pre- and postrun tool calibration procedures Sampling increments
    • Basics • • • • • • • • • 11 Repeat sections to be performed Data items and format to be included in the log header Procedures for numbering and splicing different runs in a hole Scales to be used in the presentation of logs Format and media required for digital data Requirements for reporting of time breakdown of logging operation, personnel on site, serial numbers of tools used, inventory of explosives, and radioactive sources Specific safety procedures to be followed Provision of backup tools Fishing equipment to be provided Generally speaking, the more the oil company specifies its requirements, the better. Having a strict system in place for controlling the logging operation and presentation of results ensures a smooth operation and results in high-quality data that will be consistent with previous runs. 1.4 PREPARING A LOGGING PROGRAM At the FDP stage in a field development, the outline of the logging strategy should be developed. Based on the type of well being proposed, decisions have to be made about whether to go principally for an LWD type of approach or conventional wireline and about the types of tool to be run. In general, early in the life of a field, particularly during the exploration phase, data have a high value, since they will be used to quantify the reserves and influence the whole development strategy. Moreover, lack of good-quality data can prove to be extremely expensive, particularly for offshore developments, if facilities are designed that are either too big or too small for the field. Later in field life, particularly in tail-end production, where much of the log data will not even be used for updating the static model, since it is influenced by depletion effects, the value of data becomes much less. However, even in mature fields, it may be the case that extensions to the main accumulation are still being discovered, and existing assumptions such as those regarding the position of the FWL may need to be locally revised. The FDP should lay down the broad strategy for data acquisition, which will take into account the relevant uncertainties remaining in the STOIIP and the options for adapting the wider development strategy. It is obvi-
    • 12 Well Logging and Formation Evaluation ously important that discussion take place between the petrophysicist and the geologist about the need for coring and the various analyses that will be performed once the core is recovered. For a particular well, the detailed logging requirements will first be specified within the well proposal, which will be agreed upon with other partners and any government supervisory bodies. The proposal will typically not specify the exact models of tool to be run but will cover the general types for the individual hole sections. Typically there will be many items that are conditional on hydrocarbons being encountered, based on shows encountered while drilling. For a well proposal, a typical program might look something like this: Exploration Well 171/2≤ hole section: GR/resistivity/sonic (GR to surface) If shows encountered include GR/density/neutron and optional pressure/ fluid sampling 121/4≤ hole section: LWD GR/resistivity Wireline GR/resistivity/density/neutron Optional pressure/fluid sampling if hydrocarbons encountered 81/2≤ hole section: LWD GR/resistivity GR/resistivity/density/neutron GR/dipole sonic/formation imager Pressure/fluid sampling (sampling dependent on oil being encountered) VSP Sidewall samples Development Well 171/2≤ hole section: No logs required 121/4≤ hole section: MWD [measurement while drilling]/GR GR/resistivity/sonic (GR to surface) If shows encountered include GR/density/neutron and optional pressure/ fluid sampling
    • Basics 13 81/2≤ hole section: GR/resistivity/density/neutron Dipole sonic/formation imager Pressure/fluid sampling (sampling dependent on oil being encountered) VSP Sidewall samples Note that it is not usually necessary to specify that the SP will be run, since this service is usually provided free and the log will be included in the first toolstring in the hole by default. Likewise, thermometers are usually run in the toolstring as standard, and the maximum temperature recorded is included in the log header. Prior to the actual logging job in each section, a program is usually sent to the rig with the following more detailed specifications: • • • • The actual mnemonics of tools to be run (dependent on the contractor) Intervals to be logged if different from the total openhole action How the tools are to be combined to form the individual toolstrings Data transmission/delivery requirements For the so-called conventional logs (i.e., GR, resistivity, sonic, density, neutron), it is not usually necessary to be very specific, since the company will have already established the tool parameters via generalized guidelines, as discussed in Section 1.3 above. However, the type of resistivity tool to be used will depend on the drilling mud in the hole and the resistivities expected to be encountered. While only induction tools may be run in OBM, the optimum tool to be run in WBM will depend on the ratio of the mud filtrate resistivity (Rmf) to the formation, or water, resistivity (Rw). As a rule of thumb, an induction tool is preferred if the ratio of Rmf to Rw is more than 2. However, laterolog tools tend to be more accurate in highly resistive formations (resistivity at room temperature >200 ohmm) and are inaccurate below about 1 ohmm. Induction tools, on the other hand, become saturated above 200 ohmm but are more accurate in low-resistive formations. For formation imaging, resistivity tools cannot be used in OBM, although they are definitely preferred in WBM. When OBM is used, it is necessary to use an ultrasonic device. Usually the stations to be used for pressure/fluid sampling, VSP, and sidewall sampling will be dependent on the analyses made on the first run(s) in the hole. These stations may in some cases be picked on the wellsite by the company’s representative but are usually determined in the
    • 14 Well Logging and Formation Evaluation client’s office. Therefore, a second program is usually sent to detail the further logging requirements. 1.5 OPERATIONAL DECISIONS While the logging program will aim to cover most eventualities during the logging job, often instant decisions have to be made where it is not possible to call everyone into a meeting and get the approval of all parties concerned. Below are some of the things likely to happen and some considerations in decision making. 1.5.1 Tool Failures The standard procedure if a tool fails is to replace it with a backup and carry on as before. In general, if the program has specified that a tool is to be run over a specified interval (particularly a reservoir interval), then it is best to ensure that good-quality data are acquired, even if it means making additional runs. However, the following situations may arise: • • • • When logging with LWD and in the hole drilling near the end of a section, it may be far more cost-effective, if the data are not critical, to simply carry on drilling and reacquire the data during a check trip. In some cases, the memory data may still be usable. If a wireline tool starts to act erratically, it may not justify rerunning the tool, since the data may not be critical over the particular interval and can sometimes be corrected by postprocessing the raw tool data at the surface. If an advanced tool fails, it sometimes happens that no backup tool is available on the rig. The choices then are to either try and repair the tool, arrange a backup tool to be sent from another location, or replace the tool with an earlier version with less capability. Most commonly, the latter option is chosen. In the event of failures occurring with potential safety implications (e.g., explosive charges going off accidentally), it is normal for operations to be suspended until a full investigation has been carried out to establish the cause. 1.5.2 Stuck Tools Fairly regularly during logging operations, tools get stuck in the hole, either temporarily or permanently. There are often indications of bad hole
    • Basics 15 conditions during drilling, and the logging program may be adjusted to take this into consideration. In general it is found that the longer a hole is left open, the greater the likelihood of hole problems occurring. Three sorts of sticking are found to occur: • • • differential sticking, key seating, and holding up. Differential sticking occurs when either the cable or the toolstring becomes embedded in the borehole wall and gets held in place by the differential pressure between the mud and the formation. In such a situation, it is impossible to move the toolstring either up or down. The usual procedure is to alternately pull and slack off on the tool, pulling up to 90% of the weakpoint of the cable (that point at which the cablehead will shear off the top of the toolstring). Strangely enough, this procedure is often successful, and a tool may become free after 30 minutes or so of cycling. Key seating occurs when a groove is cut into one side of the borehole, which allows the cable, but not the toolstring, to pass upward. The toolstring is effectively locked in place at a certain depth. Unfortunately this often means that when the weakpoint is broken, the toolstring will drop to the bottom of the hole and may be hard to recover or may be damaged. Holding up occurs when a constriction, blockage, dogleg, or shelf occurs in the borehole such that the toolstring may not pass a certain depth, although it can be retrieved. The usual practice in such a situation is to pull out of the hole and reconfigure the tool in some way, making the toolstring either shorter or, in some cases, longer in an attempt to work past the holdup depth. Once it is no longer possible to recover a tool on wireline, there are two options: cutting-and-threading or breaking the weakpoint. In the cut-andthread technique, the cable is cut at the surface. The drillpipe (with a special fishing head called an overshot) is run into the hole with the cable being laboriously threaded through each stand of pipe. At a certain depth it may be possible to install an SES, which means that when the toolstring is being recovered, log data may be acquired in a similar fashion to pipeconveyed logging. If the weakpoint has been broken (either accidentally or on purpose), cutting-and-threading is not possible. Then the pipe is run into the hole until the toolstring is tagged, although often the tool will drop to the bottom of the hole before it can be engaged by the overshot. Most oil companies will specify that they do not wish to break the weakpoint on purpose, even if the cut-and-thread technique is much
    • 16 Well Logging and Formation Evaluation slower. This is particularly true where nuclear sources have been lost in the hole, and every effort should be made to ensure that they are recovered undamaged at the surface. In the event of nuclear sources being lost irretrievably in the hole, which occasionally happens (particularly with LWD tools), there are usually special procedures to be followed involving notification of government bodies and steps taken to minimize the risk of and monitor any potential nuclear contamination that may occur. 1.6 CORING 1.6.1 Core Acquisition Particularly during the exploration phase of a field, coring presents an important means to calibrate the petrophysical model and gain additional information about the reservoir not obtainable by logs. Usually the decision of when and where to core will be made in conjunction with the geologist and operations department, taking into account the costs and data requirements. Generally speaking, it is considered essential to at least attempt to core a part of the main reservoir formation during the exploration and appraisal phases of drilling. A so-called conventional core will usually consist of multiples of 18 m and be 4 in. in diameter. The outer barrel has a diameter of 63/4 in. It is acquired while drilling using a metal sleeve into which the core passes during drilling. At the end of coring, the core barrel is retrieved at the surface and the core recovered from the barrel and laid out in 3-ft sections in core boxes for initial assessment on the wellsite and then transportation to the designated core laboratory. Special techniques may sometimes be proposed to improve the quality of the core and to preserve the in-situ fluids. These include: • • • • • Using a large-diameter core (5 in.) Using a fiberglass or aluminium inner sleeve, which may be cut into sections at the surface, thereby preserving the core intact within the sleeve Sponge coring, whereby a polyurethane material surrounds the core in the inner sleeve, thereby absorbing and retaining any formation fluids Resin coring, whereby a special resin is injected onto the surface of the core to seal the fluids inside Freezing the core as soon as it reaches the surface in order to preserve the fluids inside
    • Basics • • 17 Cutting plugs from the core at the wellsite, which may be sealed and used to measure the formation fluids Using tracers in the mud to attempt to quantify the extent of invasion of drilling fluid If samples have been obtained and preserved so that it is expected that the in-situ fluids are representative of the formation, the following techniques may be applied: • • Centrifuging of samples to produce formation water, which can be analyzed for chemical composition and electrical properties Applying Dean-Stark analysis to determine the relative amounts of water and hydrocarbons, thereby producing a measurement of Sw 1.6.2 Conventional Core Analysis As soon as possible after drilling, sections of the core (typically 0.5 m every 10 min) are sealed and kept as preserved samples. The remaining whole core is typically cleaned, slabbed, and laid out so that the geologist and petrophysicist can visually inspect the core and examine any sedimentary features. Important information the petrophysicist can learn from such an inspection include: • • • • • • The homogeneity of the reservoir and any variations that are likely to be below the resolution of logging tools The type of cementation and distribution of porosity and permeability The presence of hydrocarbons from smell and appearance under ultraviolet (UV) light. Oil/water contacts (OWCs) can sometimes be established in this way The types of minerals present Presence of fractures (either cemented, natural, or drilling induced) and their orientation Dip features that may influence logging tools’ response After slabbing, the usual procedure is for conventional plugs (typically 0.5 in. diameter) to be cut at regular intervals. The plugs are then cleaned by refluxing with a solvent for 24 hours and dried at a temperature that will remove any water (including clay-bound water). These plugs are then measured for porosity (using a helium porosimeter), horizontal
    • 18 Well Logging and Formation Evaluation permeability, and grain density. Additional plugs are cut in the axis of the core to determine vertical permeability. Usually a gamma ray detector or density-type device is run over the whole length of the core in order to provide a reference log that may be correlated to the wireline data. Since the “driller’s depths” to which a core is referenced are typically different from “logger’s depths,” as measured by wireline, it is necessary to make a shift before the core may be compared to logs. The conventional-plug measurements are usually performed at ambient conditions (or sometimes a few hundred psi confining pressure) and therefore need to be corrected to in-situ conditions before they may be compared to the logs. The correction factors to be used are determined through further special core analysis (SCAL). 1.6.3 Special Core Analysis SCAL measurements are typically performed on a special set of largerdiameter (1.5 in.) plugs cut from the core. These may be cut at a regular sampling increment, or the petrophysicist may specify certain depths based on the results of the conventional analyses. The most important criterion is obviously to obtain a broad spectrum of properties that fully encompass the range of properties seen in the reservoir. In order to ensure that the SCAL plugs are homogeneous, it is normal procedure to subject the plugs to a CAT (computed axial tomography) scan prior to using them for future measurements. It is hard to say how many SCAL plugs are required for a typical program, since this depends on the reservoir type, thickness, and homogeneity. In general a SCAL program may use between about 5 and 50 plugs. While many measurements are possible on core plugs, I will concentrate on the ones that are of direct relevance to the petrophysical model. These are: • • Porosity and permeability at overburden conditions. Here it is important to state the pressures at which the measurements should be performed. In Chapter 7 the equations are given for calculating the equivalent isostatic stress at which the measurements should be performed to be equivalent to in-situ conditions. Typically measurements are made at five pressures that will encompass the likely range of pressures to be encountered during depletion of the reservoir. Cementation exponent (m). In this measurement, the resistivity of the plugs is measured when they are 100% saturated with brine represen-
    • Basics • • 19 tative of the formation salinity. This measurement is usually performed at ambient conditions but may also be performed at in-situ pressure. Saturation exponent (n). In this measurement, the resistivity of the plugs is measured as a function of water saturation, with the resistive fluid being either air or kerosene. This measurement is usually performed at ambient conditions. Capillary pressure (Pc). The saturation of a nonwetting fluid (either air, mercury, or kerosene) is measured as a function of Pc applied. In a drainage cycle, 100% brine is gradually replaced by the nonwetting fluid. For an imbibition cycle (following a drainage cycle), brine is reintroduced to replace the nonwetting phase. Different techniques are available to make these measurements. In the traditional approach, m, n, and Pc would be measured using the porous plate method, with air as the nonwetting phase. Since the measurement is limited to 100 psi, additional Pc measurements would be performed using mercury injection up to 60,000 psi, thereby also determining the pore-size distribution. Many oil companies no longer favor these measurement techniques for the following reasons: 1. Measurements using mercury involve destruction of the plugs and present a potential environmental/health hazard. 2. Pc measurements involving air/mercury are not representative of true reservoir conditions and may give misleading results. 3. Porous plate measurements are slow and involve the repetitive handling of the samples to measure the saturations using a balance. If grain loss occurs, then the results are inaccurate and the electrical measurements tend to be operator dependent. Preferred techniques for undertaking these measurements are as follows: • • Measurement of m and n should be performed using a continuous injection apparatus. While not steady state, this technique has been shown to give reliable results. In the procedure, the sample is mounted vertically, flushed with brine, then kerosene-injected at a continuous rate while the resistivity and saturation are continually monitored. Pc should be measured using a centrifuge capable of up to 200 psi pressure. The sample is flushed with brine, then the amount of fluid expelled
    • 20 Well Logging and Formation Evaluation at different rotational speeds (equivalent to different pressures) is measured. This technique also has the advantage that the sample if not handled during the experiment. 1.6.4 Limitations of Core Measurements There is a tendency among petrophysicists to treat measurements made on cores as “gospel” and not to question the reservoir parameters so derived in their petrophysical model. The following may give reasons why the core data are not always correct: • • • • • A core is a section of rock cut usually over only a subset of the reservoir in a particular part of a field. There is no a priori reason why it should be representative of the reservoir as a whole. In particular, a core cut in the water leg, where diagenetic processes may be occurring, is not necessarily representative of the oil or gas legs in a reservoir. The coring and recovery process subjects the rock to stress and temperature changes that may profoundly affect the rock structure. The plugging, cleaning, and drying process may completely change the wettability of the plugs, making them unrepresentative of downhole conditions. Resistivity measurements performed on plugs at ambient temperature, using air as the nonwetting fluid, may be wholly unrepresentative of reservoir conditions. Apart from the fact that the brine has a totally different resistivity at ambient temperatures, there may be other factors affecting how easily the nonwetting phase may mingle with the wetting phase. In fact, where experiments have been performed to measure m and n under truly in-situ conditions, it was found that the values differed completely from those measured under ambient conditions. When measurements are made on a selection of, say, 10 SCAL plugs, it will typically be found that the m, n, and Pc behavior of all 10 will be completely different. These are usually then averaged to obtain a representative behavior for the reservoir. However, because of the variability, if a new set of 10 plugs is averaged, the result will be completely different. This calls into question the validity of any average drawn from 10 plugs that are taken to represent thousands of acre-feet of reservoir. Overall, it is my conclusion that it is better to use core-derived values than nothing at all, and a lot of valuable information about the reservoir can be gained from core inspection. However, no core-derived average
    • Basics 21 should be treated as being completely reliable, and there will be many cases in which it has to be disregarded in favor of a commonsense approach to all the other sources of information. 1.7 WELLSITE MUD LOGGING During the drilling of a well there will typically be a mud-logging unit on the rig. This unit has two main responsibilities: 1. To monitor the drilling of the parameters and gas/liquids/solids returns from the well to assist the drilling department in the safety and optimization of the drilling process 2. To provide information to the petroleum engineering department that can be used for evaluation purposes Typically the mud-logging unit will produce a daily “mud log,” which is transmitted to the oil company office on a daily basis. Items that will be included are: • • • • • Gas readings as measured by a gas detector/chromatograph A check for absence of poisonous gases (H2S, SO2) A report of cuttings received over the shale shakers, with full lithological descriptions and relative percentages ROP Hydrocarbon indications in samples The mud log may be of great use to the petrophysicist and geologist in operational decision making and evaluation. Areas in which the mud log may be particularly important include: • • • • Identification of the lithology and formation type being drilled Identification of porous/permeable zones Picking of coring, casing, or final drilling depths Confirmation of hydrocarbons being encountered and whether they are oil or gas 1.7.1 Cuttings Descriptions The mud-logging unit will generally take a sample of the cuttings received over the shale shakers at regular time intervals, calculated to cor-
    • 22 Well Logging and Formation Evaluation respond to regular changes in formation depth (e.g., every 5 m). Some of these samples are placed into sealed polythene bags as “wet samples” and retained. Other samples are washed, dried, and retained as “dry samples.” Washed samples are examined under a microscope in the mud-logging unit and a description made that may be communicated to the office. In order for the information received from the rig to be useful, it is essential that rigid standards for reporting are followed that are agreed upon between the rig and the office. Standards will typically vary among companies. Items that should be included are: • • • Grain properties ᭿ Texture (muddy/composite) ᭿ Type (pelletoid/micropelletoid) ᭿ Color ᭿ Roundness, or sphericity ᭿ Sorting ᭿ Hardness ᭿ Size ᭿ Additional trace minerals (e.g., pyrite, calcite, dolomite, siderite) ᭿ Carbonate particle types ᭿ Skeletal particles (fossils, foraminifera) ᭿ Nonskeletal particles (lithoclasts, aggregates, rounded particles) ᭿ Coated particles Porosity and permeability ᭿ Porosity type (intergranular, fracture, vuggy) ᭿ Permeability (qualitative as tight, slightly permeable, highly permeable) Hydrocarbon detection Hydrocarbons may be detected with one of the following methods: Natural fluorescence Examining the cuttings under UV light may indicate the presence of oil, since oil will fluoresce. However, fluorescence will not in itself prove the presence of movable oil, due to other sources of fluorescence that may be present, such as fluorescent minerals; OBM or lubricants used; other sources of carbon, such as dead oil or bitumen; and Gilsonite cement. The correct procedure is for a portion of the lightly washed and undried cuttings to be placed on a dish and observed under UV light (other light
    • Basics 23 sources having been removed). Those parts of the sample exhibiting fluorescence are picked out and placed in a porcelain test plate hole to be examined for cut fluorescence. Solvent cut To measure the solvent cut, about 3 cm of dried and crushed sample is placed in a test tube and solvent is added to about 1 cm above the sample. The test tube is shaken for a few minutes, then left to stand. The solvent cut is the change of coloration of the solvent. Solvents that are commonly used are chlorothene, ether, and chloroform. Precautions are required in handling these solvents, since they are toxic and flammable. Heavy oils generally give a stronger cut than lighter ones. Asphalts will therefore give a stronger cut than paraffins. Condensate gives rise to only a very light cut. In addition to the cut, a residual oil ring may be observed around the test tube after the solvent has evaporated. In solvent cut fluorescence, the cut fluorescence is measured by taking the test tube used for the solvent cut and placing it under UV light together with a sample of the pure solvent (to check for possible contamination) and observing whether any fluorescence is present. Acetone test The acetone test involves placing a sample of washed, dried, and crushed cuttings in a test tube with acetone. After shaking, the acetone is filtered into another test tube and an equal amount of water added. Since acetone is dissolvable in water but hydrocarbons are not, the liquid becomes milky in color. This test is particularly useful where light oil or condensate is present and there is no other source of carbon in the samples. Visible staining Particularly if the permeability and/or viscosity is poor, oil may remain in cuttings and be visible under the microscope in the form of a stain on the surface of the cutting. Odor The characteristic smell of oil may sometimes be discerned during the cleaning and drying process.
    • 24 Well Logging and Formation Evaluation Gas detection analysis Gas detectors work by passing air drawn from where the mud reaches the surface (the bell nipple) over a hot detector filament. This combusts the gas, raising the temperature and lowering the resistance of the filament. At high voltages all the combustible gases burn, whereas at lower temperatures only the lighter components burn. By recording the change in resistance at different voltages, the relative proportions of the various components may be estimated. A gas chromatograph may also be used to further differentiate the various hydrocarbon components. Particularly for the detection of poisonous gases, such as H2S, Drager tubes may be used on the rig floor. 1.8 TESTING/PRODUCTION ISSUES At the end of the logging, a decision will have to be made as to whether casing should be run or not. If the well results are not as expected, there may be an immediate decision required to either sidetrack or abandon the well. Therefore, a quick but accurate interpretation of the data, not always made using a computer, is of primary importance. If the decision is made to test or complete the well, the petrophysicist will also be required to pick the perforation intervals. A few points to bear in mind here are that when picking the intervals from a log it is important to specify the exact log being used as a depth reference. Since depths on field prints are sometimes adjusted to tie in with previous runs when the final prints are made, confusion can occur. The safest thing is to include a photocopy of the reference log, with the intervals to be perforated marked on the log, along with any program passed to the rig. The correct procedure for ensuring that the well is perforated “on depth” with wireline operations is as follows: 1. Initially it is necessary to tie the depths of casing collars, as measured in the well using a CCL, with the reference openhole GR log. This is done by making a run in the hole with a GR/CCL tool and comparing the depth with the openhole reference log on which the perforated interval has been marked. 2. The print of the GR is then adjusted so that the depths of the casing collars are on depth with the openhole GR.
    • Basics 25 3. When the perforating guns are run, they will be combined with only a CCL log. On first running in the hole, the CCL on the perforating guns will be off depth with the corrected GR/CCL. 4. Because of irregularities in the length of joints of casing, the CCL acquired with the guns may be overlain on the GR/CCL and a unique fit made. This enables the logger’s depths of the perforating guns to be adjusted so that the CCL is on depth with the GR/CCL (which is itself on depth with the reference openhole log). Obviously if all the joints of casing were the same length, it would be possible to find a fit when the toolstring was off depth by the length of one casing joint. This problem may be avoided by running a radioactive pip tag as part of the completion, which enables the CCL on the perforating gun to be tied with certainty to the GR/CCL. 5. Once the gun has been fired, there may be indications on the surface, such as changes in cable tension. After a few minutes, there may also be indications of an increase in tubing head pressure. The most appropriate guns, charges, phasing, and well conditions (fluid type and drawdown) all need to be considered. Usually the contractor is able to offer advice on this and should be involved in any meetings at which the perforation procedures are to be determined. Wherever possible it is best that wells be perforated “under drawdown.” This means that the pressure in the wellbore is lower than the formation pressure, which ensures that the well is able to flow as soon as perforation has occurred and avoids the risk of either completion fluid or debris blocking the perforations. I have seen many cases in which the petrophysicist has picked many short intervals to be perforated, separated by only a foot or so. Since the accuracy of depth correlation is never perfect, it is sometimes advisable to shoot a continuous section, which includes parts that are not of the reservoir. There has always been caution about perforating shales, in case they result in “fines” being produced. I can only say that I have never heard of this occurring in practice. In general I favor perforating as much of the potentially producible interval as possible (a safe distance from any water-bearing sands). If you look at the economics of a well, an additional 10 bbl/day over the life of a well will result in a far greater economic benefit than the additional cost of perforating an extra 10 m. In some cases, picking too short an interval may result in the well never even managing to flow to surface when otherwise it could be an economic producer. It may frequently occur, particularly in depleted reservoirs, that the well
    • 26 Well Logging and Formation Evaluation doesn’t produce as it was supposed to, as per calculations. Either it fails to produce at all, it dies quickly, or water or gas breakthrough occurs. Very often the petrophysicist is called to provide an explanation for these phenomena. When a well fails to flow or dies very quickly, the first thing to look at is the perforation operation. Check the following: • • • • • Is the petrophysical interpretation completely reliable? What was the kh as derived from the logs? Could formation damage have occurred during the cementation and completion operation? Is there proof that the guns detonated, and were they on depth? What was the over/underbalance at the time of perforation? What drawdown is currently being applied? Are there any other mechanical factors (sliding side doors, safety valves) that could prevent flow? When water breakthrough occurs sooner than expected, the following should be considered: • • • • • • • How close are the perforations to any water leg as logged in the well? Could water coning be occurring? What is the quality of the cement bond, and could there be flow behind casing? Where does the dynamic model of the reservoir predict the encroaching water front to be? Are neighboring wells already starting to water out, and are TDT/GST data available for any? Could the water be entering from elsewhere in the wellbore (other producing zones, leaks in the tubing, etc.)? What does the relative permeability data from the core indicate the relative permeability to oil and water to be? Is the well in the transition zone, and how sure are you that any Sw calculated on the logs is capillary or clay bound? When gas is produced unexpectedly, the following should be considered: • • How close are the perforations to any gas leg as logged in the well? Could gas cusping be occurring? What is the quality of the cement bond, and could there be flow behind the casing?
    • Basics • • • • • 27 Where does the dynamic model of the reservoir predict the GOC to be? Is the bottomhole pressure below the bubble point? What choke size is being used? Are neighboring wells already starting to produce gas? Could the gas be entering from elsewhere in the wellbore (other producing zones, leaks in the tubing, etc.)? What does the relative permeability data from the core indicate the relative permeability to oil and gas to be? Once the above questions have been answered for the various scenarios, the petrophysicist will be in a good position to contribute to discussions regarding remedial actions to be taken. Such actions may include any of the following: • • • • • • • • • • Further data gathering through the use of production logs Reperforation Acidization Acid or hydraulic fracturing of the reservoir Sealing off of certain zones either chemically or mechanically Modification of the offtake strategy Recompletion Sidetracking of the well Implementation of artificial lift techniques (e.g., gas lift or ESP [electric submersible pump]) Implementation of a water or gas injection program
    • C H A P T E R 2 QUICKLOOK LOG INTERPRETATION Once the section TD (total depth) of the hole has been reached, the petrophysicist will be expected to make an interpretation of the openhole logs that have been acquired. Before starting the log interpretation, the petrophysicist should have: 1. All the relevant daily drilling reports, including the latest deviation data from the well, last casing depth, and mud data 2. All the latest mud-log information, including cuttings description, shows, gas reading, and ROP (rate of penetration) 3. Logs and interpretations on hand from nearby wells and regional wells penetrating the same formations, in particular where regional or fieldwide values of m, n, Rw, rhog and fluid contacts are available 4. A copy of the contractor’s chart book 2.1 BASIC QUALITY CONTROL Once the log arrives, the petrophysicist needs to ensure the quality of the log data and should perform the following regimen: 1. Check that the logger’s TD and last casing shoe depths roughly match those from the last daily drilling report. 2. Check that the derrick floor elevation and ground level (or seabed) positions are correct. 3. Check that the log curves are on depth with each other. The tension curve can be used to identify possible zones where the toolstring has become temporarily stuck, which will put the curves off depth and result in “flatlining.” 29
    • 30 Well Logging and Formation Evaluation 4. Check that the caliper is reading correctly inside the casing (find out the casing ID) and that it is reading the borehole size in nonpermeable zones that are not washed out. 5. Check the density borehole correction curve. It should not generally exceed 0.02 g/cc, except in clearly washed out sections (>18 in.), for which the density curve is likely to be unusable. 6. Inspect the resistivity curves. If oil-based mud (OBM) is being used, the shallow curves will usually read higher than the deep curves (except in highly gas or oil saturated zones). Likewise, with waterbased mud (WBM) the shallow curves will read less than the deep curves, providing Rmf < Rw, or in hydrocarbon-bearing zones. In theory, the curves should overlie each other in nonpermeable zones such as shales. However, in practice this is often not the case, due to either anisotropy or shoulder-bed effects. 7. Check the sonic log by observing the transit time in the casing, which should read 47 ms/ft. 8. Look out for any cycling-type behavior on any of the curves, such as a wave pattern. This may be due to corkscrewing while drilling, causing an irregular borehole shape. However, it is necessary to eliminate any possible tool malfunction. 9. Check that the presentation scales on the log print are consistent with other wells or generally accepted industry norms. These are generally: ᭿ GR: 0–50 API ᭿ Caliper: 8–18≤ ᭿ Resistivity: 0.2–2000 ohmm on log scale ᭿ Density: 1.95–2.95 g/cc (solid line) ᭿ Neutron: -0.15 ± 0.45 (porosity fraction) (dashed line) ᭿ Sonic: 140–40 ms/ft 2.2 IDENTIFYING THE RESERVOIR For the next section of this chapter, it will be assumed that one is dealing with clastic reservoirs. Carbonates and complex lithologies will be discussed later in the book. The most reliable indicator of reservoir rock will be from the behavior of the density/neutron logs, with the density moving to the left (lower density) and touching or crossing the neutron curve. In clastic reservoirs in nearly all cases this will correspond to a fall in the gamma ray (GR) log. In a few reservoirs, the GR is not a reliable indicator of sand, due to the presence in sands of radioactive minerals. Shales
    • 31 Quicklook Log Interpretation can be clearly identified as zones where the density lies to the right of the neutron, typically by 6 or more neutron porosity units. The greater the crossover between the density and neutron logs, the better the quality of the reservoir. However, gas zones will exhibit a greater crossover for a given porosity than oil or water zones. Because both the neutron and density logs are statistical measurements (i.e., they rely on random arrivals of gamma rays in detectors), they will “wiggle” even in completely homogeneous formations. Therefore, it is dangerous to make a hard rule that the density curve must cross the neutron curve for the formation to be designated as net sand. For most reservoirs, the following approach is safer (see Figure 2.2.1): 1. Determine an average GR reading in clean sands (GRsa) and a value for shales (GRsh). For GRsh, do not take the highest reading observed, but rather the mode of the values observed. 0.45 GR (API) 0 Neutron (frac) Density (g/cc) 150 1.95 5000 5005 5010 Depth 5015 5020 5025 5030 5035 Figure 2.2.1 Identifying Net Reservoir –0.15 2.95
    • 32 Well Logging and Formation Evaluation 2. Define the shale volume, Vsh, as (GR - GRsa)/(GRsh - GRsa). By comparing Vsh with the density/neutron response, determine a value of Vsh to use as a cutoff. Typically 50% is used. If the GR is not usable as a sand indicator, then for now just treat the entire gross as being net sand and apply a porosity cutoff at a later stage (see next section). 2.3 IDENTIFYING THE FLUID TYPE AND CONTACTS Because the porosity calculation will depend on the formation fluid type, it is good at this stage to at least have a working assumption regarding the fluids. If regional information is available regarding the positions of any gas/oil contact (GOC) or oil/water contact (OWC), then convert these subsea depths into measured depths in the current well and mark them on the logs. If the formation pressures have already been measured (this is usually never the case), then any information on possible free water levels (FWLs) or GOCs can also be marked on the log. Start by comparing the density and deepest reading resistivity log for any evidence of hydrocarbons. In the classic response, the resistivity and density (and also GR) will be seen to “tramline” (i.e., follow each other to the left or right) in water sands and to “Mae West” (i.e., be a mirror image of each other) in hydrocarbon sands. However, some hydrocarbon/water zones will not exhibit such behavior, the reasons being: • • • • • When the formation-water salinity is very high, the resistivity may also drop in clean sands. In shaly sand zones having a high proportion of conductive dispersed shales, the resistivity may also fail to rise in reservoir zones. If the sands are thinly laminated between shales, the deep resistivity may not be able to “resolve” the sands, and the resistivity may remain low. If the well has been drilled with very heavy overbalance, invasion may be such as to completely mask the hydrocarbon response. When the formation water is very fresh (high Rw), the resistivity may Mae West even in water-bearing zones. When either of the first two situations arises, it is very important to look at the absolute value of the deep resistivity, rather than at only the behavior compared with the density. As long as a known water sand has been
    • Quicklook Log Interpretation 33 penetrated in the well (or a neighboring well), one should already have a good idea of what the resistivity ought to be for a water-bearing sand. If the resistivity is higher than this value, whatever the shape of the curve, then hydrocarbons should be suspected. Obviously any mud-log data (gas shows, fluorescence) should be examined in the event that it is not clear whether or not the formation is hydrocarbon bearing. However, the mud log can certainly not be relied on to always pick up hydrocarbons, particularly where the sands are thin and the overbalance is high. Moreover, some minor gas peaks may be observed even in sands that are water bearing (Figure 2.2.2). As stated earlier, gas zones will exhibit a greater density/neutron crossover than oil zones. In a very clean porous sand, any GOC can be identified on the log relatively easily. However, in general, GOCs will be identified correctly in only about 50% of cases. Secondary gas caps appearing in depleted reservoirs will usually never be picked up in this way. Formation-pressure plots represent a much more reliable way to 0.45 GR (API) 0 150 1.95 Neutron (frac) –0.15 0.1 Redeep (ohmm) 1000 Density (g/cc) 2.95 0.1 5000 5005 5010 Depth 5015 5020 5025 5030 5035 Figure 2.2.2 Identifying Net Pay Rshallow (ohmm) 10 1000
    • 34 Well Logging and Formation Evaluation 0.8 0.7 0.6 Vp /Vs 0.5 gas sands oil sands 0.4 0.3 0.2 0.1 0 4900 5000 5100 5200 5300 5400 5500 Vp (m/s) Figure 2.2.3 Identifying Gas Sands from Vp/Vs vs. Vp plot identify GOCs, but these will generally be useful only in virgin reservoirs. Various crossplots have been proposed in the past involving the GR, density, neutron, and sonic logs as a way to identify gas zones, but I have never found these to be reliable. In a depleted reservoir where gas has started to come out of solution in an oil zone and not had a chance to equilibrate (i.e., form a discrete gas cap), the gas may exist in the form of football-sized pockets surrounded by oil. In such a situation the basic logs will never give a definitive answer. The most reliable way I have found to identify gas zones is to use the shear sonic log (if available) combined with the compressional sonic. If compressional velocity (Vp)/sheer velocity (Vs) is plotted against Vp, then because Vp is much more affected by gas than is Vs, a deviation will be observed in gas zones (Figure 2.2.3). 2.4 CALCULATING THE POROSITY Porosity should be calculated from the density log using the equation: f = ( rho m - density) ( rho m - rho f ) where rhom = matrix density (in g/cc) and rhof = fluid density (in g/cc).
    • 35 Quicklook Log Interpretation Table 2.4.1 Selection of fluid density for porosity calculated from density tool WBM Formation Fluid OBM Heavy Mud System Light Mud System Gas, clear gas effect on logs Gas, no clear gas effect on logs Light oil Heavy oil Low-salinity water High-salinity water 0.4 0.55 0.6 0.7 0.85 0.9 0.6 0.7 0.8 0.9 1.05 1.1 0.5 0.6 0.7 0.8 1.0 1.05 The density tool actually works by injecting gamma rays into the formation that are then scattered by electrons in the formation, a process known as Compton scattering. These gamma rays are then detected by two detectors. Since the tool actually measures electron density, there is a slight miscalibration due to the variation in electron density between different minerals. The correction is typically small (typically 1% or less), so is no major cause for concern. Assuming that the density porosities will at some stage be calibrated against core data, this correction can be ignored, at least for quicklook purposes. For sandstones, rhom typically lies between 2.65 and 2.67 g/cc. Where regional core data are available, the value can be taken from the average measured on conventional core plugs. Fluid density, rhof, depends on the mud type, formation fluid properties, and extent of invasion seen by the density log. Table 2.4.1 gives some typical values that may be used. As to the appropriateness of the values being used, the following tests may be applied: • • • Where regional information is available, the average zonal porosities may be compared with offset wells. In most cases, there should be no jump in porosity observed across a contact. An exception may occur across an OWC where diagenetic effects are known to be occurring. In no cases in sandstones would one expect porosities to exceed about 36%. Note that the porosity calculated from the density log is a total porosity value; that is, water bound to clays or held in clay porosity is included.
    • 36 Well Logging and Formation Evaluation This has the advantage, therefore, of being directly comparable to porosities measured on core plugs, since these have had all clay-bound and free water removed. Having calculated the porosity, it is important to check for any zones where washouts have resulted in erroneously high density values and thus unrealistically high porosities. In some cases it is sufficient to just apply a cutoff to the data whereby porosities above a certain value are capped at a value. This recognizes the fact that zones often wash out because they are soft and have a high porosity. However, in some cases it is necessary to manually edit the density log using one’s best estimate of what the density should be. Note that in water-bearing sections a good estimate of porosity, f, may be made using true resistivity (Rt) and Archie’s equation, which is: Rt = Rw * f - m * Sw n or ( -1 n ) Sw = [( Rt Rw ) * f m ] where Rw m Sw n . = formation water resistivity (measured in ohmm) = the cementation, or porosity, exponent = water saturation = saturation exponent. Alternatively, sometimes a correlation can be made between the GR and density in non–washed-out zones and applied. I generally favor always working in a total porosity system. The term effective porosity is also used, although often different people take it to mean different things. Probably the best definition is that it is the total porosity minus the clay-bound water and water held as porosity within the clays. It may therefore be defined as: feff = ftotal * (1 - C * Vsh ) where C is a factor that will depend on the shale porosity and CEC (cation exchange capacity). It may be determined from calculating the total porosity in pure shales (Vsh = 1) and setting feff to zero. However, I have doubts about the correctness of assuming that properties of the shales in nonreservoir zones can be applied to dispersed shales within sands in the
    • Quicklook Log Interpretation 37 reservoir. In general I do not recommend calculating feff at all as part of any quicklook evaluation. At this point I would like to make it clear that I never favor making use of the neutron/density crossplot log for calculating porosity in sandstones. My reasons for this are as follows: 1. Both the neutron and density logs are statistical devices and vary randomly within certain limits determined by the logging speed, detector physics, source strength, and borehole effects. The error introduced when two such random devices are compared is much higher than when one such device is used on its own. 2. The neutron is severely influenced by the amount of chlorine atoms in the formation, occurring either in the formation water or in the clay minerals. This means that the neutron porosity is only very loosely related to the true porosity (as observed when it is compared with the density log in sand/shale sequences!). 3. The neutron is also affected in an unpredictable way by gas (unlike the density, for which a correction can be made using the appropriate rhof). 4. I have never had much faith in the overlays presented on standard neutron/density crossplots by the contractors. In practice, when real data are plotted, the overlays typically predict all kinds of minerals, from dolomite to limestone, to be present when in fact one is dealing with a clay/quartz combination. When I do a quicklook I use the neutron log for only two things: (1) qualitative identification (using the density) of shale/sand zones and (2) identification of gas zones. I also do not favor the use of the sonic log for porosity determination under any circumstances. In my view you are better off just making an informed guess at the formation porosity based on the general log response and regional information, rather than relying on any quantitative calculation based on the compressional sonic. 2.5 CALCULATING HYDROCARBON SATURATION In most quicklook evaluations of clastic reservoirs, it is sufficient to use Archie’s equation (see above) to calculate saturations, using the deepest reading resistivity tool directly as Rt. In the absence of any regional core values, I would recommend using m = n = 2. Note that I have not chosen to include the so-called Humble constant (a), since this can just as easily be incorporated within Rw.
    • 38 Rt (ohmm) Well Logging and Formation Evaluation Porosity Figure 2.5.1 Pickett Plot If m and n are predefined, then clearly the key parameter that must be determined is Rw. By far the preferred method of determining the best Rw to use in a particular well evaluation is a Pickett plot over a known waterbearing section of the formation (Figure 2.5.1). By plotting Log(Rt) vs. log(f), m may be determined from the gradient of the line drawn through the points, and Rw may be read from the intercept of the line with the Rt axis. Note that if m is fixed, the line can be moved only up and down. At this point, if the slope of the data is clearly at odds with the assumed m value, I would recommend changing m, provided that it still lies within a reasonable range (1.5–2.5). Some information regarding Rw may also be available from regional data and produced water samples in neighboring wells. Note that this will usually be in the form of a salinity expressed as NaCl concentration in ppm or mg/l. This has to be converted to an Rw value using the contractor’s chart book and knowledge of the formation temperature. Where no clear water legs have been logged in the well, there is no alternative but to use regional data, although the Pickett plot may yield a different value than that expected from regional information. Reasons for this may be one of the following:
    • Quicklook Log Interpretation • • • • 39 The porosities calculated in the well are incorrect. The zone may not in fact be 100% water bearing as assumed. The value of m needs to be adjusted. The regional value is not applicable in this well. Reasons why the regional value may not apply are: • • • • • The salinity may be different in this well. The chart books assume that the conductivity of the brine is caused only by the presence of NaCl. If other chlorides are present (e.g., MgCl), the Rw calculated from the chart book will be wrong. The samples from which the salinity has been measured in other wells may be contaminated or affected by salt dropout when the samples were recovered at surface. If shale effects are predominant, the conductivity arising from claybound water may have a different salinity from that produced in a well. Typically, clay-bound water will be fresher than free water. The water zone may have originally been oil bearing but became flushed by injection water of a different salinity (this is common offshore, where seawater is often used for injection). In theory, the spontaneous potential (SP) curve may be used in some instances to derive a measurement of Rw, although I have never had much success with it. The procedure for doing this is as follows: 1. On the SP log, draw a shale baseline, which is a line defining an average of the SP readings in 100% shale zones. 2. Determine the maximum SP deflection (in mV) from the baseline to the reading observed in thick, porous parts of the reservoir. 3. Using the appropriate chart as supplied by the contractor, convert the maximum SP deflection to a static spontaneous potential (SSP) value. This corrects for invasion, borehole, and bed effects. 4. From the appropriate chart, determine kinetic energy (Ek), mc (the mudcake contribution). 5. Calculate Ek, shale using Ek, shale = DP(bar)/6.9, where DP is the pressure difference between the mud pressure and formation pressure. 6. Calculate the Eckert number (Ec) of the bottomhole temperature (BHT) in °C: Ec(BHT ) = SSP + Ek , mc - Eksh .
    • 40 Well Logging and Formation Evaluation 7. Correct Ec(BHT) to standard temperature using: Ec(25∞C) = Ec(BHT ) * 298 (273 + BHT ). 8. Determine the mud filtrate salinity from Rmf and temperature. 9. Determine Qvshale using the appropriate chart as supplied by the contractor. Or else use Qvshale = 4 mmol/cc. 10. With the appropriate chart, use Ec(25°C), mud salinity, and Qvshale to determine the formation-water salinity. 11. Convert the formation-water salinity to Rw using the BHT. At the end of the day it is essential that the model used calculates 100% water in known virgin water-bearing reservoirs. If this is not the case, you may be certain that the Sw calculated in the reservoir will be incorrect. 2.6 PRESENTING THE RESULTS Having calculated the f and Sw curves, it is usually required to provide averages over various formation zones. This should be done as follows. First of all determine over which depths the results should be broken up. Apart from the formation boundaries as agreed upon with the geologist, further subdivision should be made for any possible changes in fluid type or zones where the data are of particularly poor quality, or at any points where there is marked change in log character. A table such as Table 2.6.1 should be produced. Note that the average porosity is given by: (f)average =  fi h , where h is the net thickness. (2.6.1) The average value of Sw is given by ( Sw )average =  fi * Swi  fi . (2.6.2) Where a permeability transform is available, the average permeability over each major sand body should also be presented. Usually the net may be defined on the basis of a Vsh cutoff. However, where this has not been possible, a porosity cutoff should be used. Generally, the cutoff point should be set at a value equivalent to a per-
    • 41 Quicklook Log Interpretation Table 2.6.1 Reporting the results of an evaluation Zone Top (m) Base (m) Gross (m) Net (m) Average Porosity Sw Zone 1, gas Zone 1, oil Zone 1, possible oil Zone 1, water Zone 2 Zone 3 Total gas zones Total oil zones meability of 1 millidarcy (md) for oil zones and 0.1 md for gas zones. In general I don’t favor the idea of cutoffs, because all too often they result in potential reserves being excluded from the calculation of STOIIP (stock tank oil initially in place) or GIIP (gas initially in place). However, since Archie’s equation will often yield nonzero hydrocarbon saturations in 100% nonreservoir shales, it is usually necessary to apply some kind of cutoff to the data. I particularly object to the practice of applying a further Sw cutoff and deriving a “pay” footage for a zone. Such a number has no place whatsoever in any kind of STOIIP or GIIP calculation. In theory, a pay footage might be used to assist in decision making regarding which zones to perforate. However, in practice this is performed more effectively by laying out a 1 :200 print of the evaluated logs and deciding on that basis which zones are worth perforating. For presentation purposes it is useful to generate a 1 :500 version of the evaluated log, with as much data included as possible. Although different companies use different conventions, it is common to use green for gas, yellow for unidentified hydrocarbon, red for oil, and blue for water zones. I would recommend generating a curve called SHPOR, derived from (1 - Sw)*Por, and include it in the porosity track, shading from 0 to the curve using the appropriate fluid color. This curve is useful because the area colored is representative of the total volume of the fluid. Hence a thin zone having a high porosity is given more prominence than a thicker zone that might have a much lower porosity.
    • 42 Well Logging and Formation Evaluation Exercise 2.1 Quicklook Exercise Using the log data presented in Appendix 1 (test1 well), do the following: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. Pick GRsa and GRsh from inspection of the logs. Calculate Vsh. Pick the likely position of the OWC. Assuming appropriate fluid densities for the oil and water legs (well was drilled with fresh WBM) and a grain density of 2.66 g/cc, calculate the porosity. Set the porosity to zero wherever Vsh > 0.5. Make a Pickett plot over the water-bearing interval. Assuming that m = n = 2, choose an appropriate Rw. Calculate Sw using Archie’s equation. Check the assumed position of the OWC. If it needs to be moved, recalculate the porosity (and Sw) accordingly. Calculate SHPOR. Display SHPOR in the same depth track as the porosity. Divide the formation into appropriate intervals and calculate sums and averages. Suggest points for the formation pressure tool. 2.7 PRESSURE/SAMPLING In most cases there will be a requirement to run the pressure/sampling tool to acquire pretests and possibly downhole samples. While these data are also used by the reservoir engineer and production technologist, they can be extremely valuable to the petrophysicist in determining the fluids present in the formation. Pretests can provide the following information: • • • • • The depths of any FWLs or GOC in the well The in-situ fluid densities of the gas, oil, and water legs The absolute value of the aquifer pressure and formation pressure A qualitative indication of mobility and permeability The bottomhole pressure and temperature in the wellbore Additionally, acquiring downhole samples can provide the following information:
    • Quicklook Log Interpretation • • • 43 Pressure/volume/temperature (PVT) properties of the oil and gas in the reservoir Formation-water salinity Additional mobility/permeability information In the conventional mode of operation, a probe is mechanically forced into the borehole wall and chambers opened in the tool into which the formation flows. Pretest chambers are small chambers of a few cubic centimeters that can be reemptied before the next pretest station. For downhole sampling, larger chambers are used, typically 23/4 or 6 gallons. Since the first fluid entering the tool is typically contaminated by mud filtrate, normal practice is to make a segregated sample; that is, fill one chamber, seal it, and then fill a second chamber (hopefully uncontaminated). Once the chambers are retrieved at surface, they may be either drained on the wellsite or kept sealed for transferring to a PVT laboratory. Optional extra modes in which the tools can typically be used include the following: 1. As an arrangement of packers in order to isolate a few meters of the borehole wall, thereby providing a greater flow area 2. As a pumpout sub while sampling in order to vent the produced fluids into the wellbore until it is hoped that the flow is uncontaminated by mud filtrate 3. To monitor the fluid properties (resistive, capacitant, optical) while pumping out to determine whether oil, water, or gas is entering the chamber 4. As dual packer assemblies run to create a “mini-interference test” that can be used to assess the vertical communication between different intervals. Pretests and sampling are often not successful. Moreover, the fact that the tool is stationary in the hole for long periods means that there is a higher than usual chance of getting the tool stuck in the hole. One of these problems can occur: • • Seal failure. The rubber pad surrounding the probe, which provides a seal between the mud pressure and the formation pressure, may fail, resulting in a rapid pressure buildup to the mud pressure. Supercharging. Tight sections of the formation may retain some of the pressure they encounter during the drilling pressure (which is higher
    • 44 • • Well Logging and Formation Evaluation than the static mud pressure). The pretest pressure is measured as a pressure that is anomalously high. Dry test. If the formation is very tight, there may be a very slow buildup of pressure in the pretest chambers, and it is not operationally feasible to attempt to wait until equilibrium is reached. Anomalous gradients. If sands are isolated even over geological time scales, then they may lie on different pressure trends, not sharing a common aquifer of FWL. Also, if any depletion has occurred in the reservoir or the reservoir is not in a true equilibrium state (for instance, due to a slowly leaking seal or fault), then gradients may not be meaningful. At this point it will probably be helpful if the distinction is explained among FWL, FOL (free oil level), OWC, GWC (gas/water contact), and GOC and how they are related in pressure measurements (see Figure 2.7.1). Pressure (psia) 4950.00 2990 4960.00 4970.00 4980.00 4990.00 5000.00 3000 3010 3020 Form Press GOC - 3047.5 OWC - 3059 FWL - 3063 gas (0.1 g/cc) Depth (mtvdss) 3030 3040 oil (0.3 g/cc) 3050 water (1.05 g/cc) 3060 3070 3080 3090 Figure 2.7.1 Example of Formation Pressure Plot
    • Quicklook Log Interpretation 45 The FWL is the point at which the capillary pressure, Pc, in the reservoir is zero and below which depth no hydrocarbons will be found within that pressure system. Often the FWL may be related to the spill point of the structure, particularly where there is an abundant supply of hydrocarbons in the system. On a formation pressure/depth plot, the intersection between the points of the oil and the water (or gas and water) will fall at the FWL. Above the FWL, Pc is available to allow the drainage of water by hydrocarbons. However, particularly in low-permeability rocks, a certain entry pressure is required before the value of Sw can fall below unity. Once this pressure is reached, hydrocarbons will be found in the rock and one can be said to be above the OWC or GWC. Note that between the FWL and the OWC/GWC, pressure points will continue to fall on a waterline. For an oil/gas reservoir, the pressure will rise above the OWC on a trend corresponding to an oil gradient (but intersecting the waterline at the FWL). At the GOC, technically one would expect some kind of similar FWL/OWC effect to occur with an FOL. However, the situation is not the same as at the OWC, because one is dealing with three phases (gas/oil/water) and not two, as before. Hence, it is common practice to treat the GOC as being the same as the intersection point of the gas and oil pressure lines. This may be technically incorrect, but I can only say that it has never caused me any problems during my career as a petrophysicist. For a gas-only reservoir, the pressure will rise above the GWC on a trend corresponding to a gas gradient (but intersecting the waterline at the FWL). Note that the above considerations have nothing to do with the “transition zone” that relates to the interval between the OWC or GWC and the point at which hydrocarbon values start to approach “irreducible” values. This will be discussed in Chapter 4. In poor-quality rocks, the effect of entry height can be appreciable (up to tens of meters). It may have the effect of causing the OWC/GWC to vary in depth across the field if the reservoir quality is changing. 2.8 PERMEABILITY DETERMINATION During a typical pretest, the pressure gauge will show a behavior as shown in Figure 2.8.1. The behavior of the pressure buildup, analogous to a production-test buildup, may be used to estimate the properties of the formation. The mobility (M) of the formation is defined by:
    • 46 Well Logging and Formation Evaluation 4000 Pressure (psia) set packer open chamber hydrostatic 3000 retract packer 2000 1000 start build-up 0 0 50 100 150 200 Time (s) Figure 2.8.1 Pressure Measurements During a Pretest Table 2.8.1 Typical viscosities of borehole fluids Fluid Water Diesel Oil Gas M = (k m ) Viscosity (cp) 0.3 2–3 3–10 0.015 (2.8.1) where k = permeability of formation, in md, and m = viscosity of fluid entering chamber, in centipoises (cp). It may be shown theoretically that the mobility of the formation is related to the drawdown pressure, drawdown time, and flow rate. From analysis of the buildup, the contractor will normally give a mobility estimate. For conversion of the mobility to permeability, the viscosity needs to be known. In most cases the pretest chamber will be filled with mud filtrate, either water or oil based. Table 2.8.1 gives some values. While pretests are very useful in that they can prove that some permeability is present if a good buildup is obtained, it should be remem-
    • Quicklook Log Interpretation 47 bered that they represent only a point measurement. Typically moving the probe up or down by a few centimeters may result in a completely different measurement of mobility. The lack of a good buildup may be purely the result of bad luck in the positioning of the probe. Moreover, the results may not give an accurate idea of the average permeability of a zone. In general, pretests should be used to verify that a zone has some permeability, but the other methods used (e.g., permeability as derived from a poroperm relationship) are to determine an average permeability to be used in dynamic models. A pretest permeability being lower than that derived from a poroperm relationship may be a result of formation damage occurring while drilling. This may also be observed when the zone is tested for production. Petrophysicists should always try to obtain the actual field print from the contractor when doing field studies, with a view to assessing permeability and fluid contacts. Reasons for this are as follows: • • • • Older-generation tools report pressures from a strain gauge, which measures psi per gauge (psig) rather than the absolute psi (psia) reported from quartz gauges. If the values are entered incorrectly into a database, there will be a shift equivalent to atmospheric pressure (14.7 psi). When databases are created for fields (e.g., a shared ExcelTM spreadsheet), sometimes not all the field data are entered, such as zones reported as “tight.” Knowledge of tight zones is crucial if zones are being considered for recompletion based on log-derived permeability estimates. When zones are reported as being tight or of limited drawdown, it may be possible in some cases to make an estimate of formation pressure by extrapolating the buildup pressures. The contractors will typically report a measured depth for the pretest, as well as a true vertical depth (TVD), with reference to the derrick floor. It is important to check that the pressures used are being referenced properly to the best estimate of TVD relative to the datum (usually mean sea level). After the pressure tool is run, there will typically be a gyro survey run once the final casing is set, and this should be used to convert all measured depths in the well to TVD relative to the datum.
    • 48 Well Logging and Formation Evaluation Exercise 2.2. Using Pressure Data 1. Using the formation pressure data points acquired (and presented in Appendix 2), calculate the formation-fluid densities and position of the FWL. Assume that the logs are TVD and measured from mean sea level. 2. Do you interpret the zone to be oil or gas bearing? 3. Given that the sand is laterally extensive, would you propose a production test in this well, and which intervals would you perforate?
    • C H A P T E R 3 FULL INTERPRETATION The quicklook analyses presented in Chapter 2 will be sufficient for operational decisions on the well. Usually the results are presented by making a clear print of the evaluated logs at scales of 1 : 200 and 1 : 500 with the sums and averages marked on the logs and the porefluids marked using appropriate colors. All companies will use blue for water, but some prefer red for oil and green for gas, while others prefer red for gas and green for oil. Once the final data and prints have been received from the logging contractor, the digital data should be stored within a corporate database. Normally at this point the petrophysicist will do a full interpretation, which might be revised as further core analysis or information from offset wells becomes available. In some cases the quicklook Archie model might be completely set aside in favor of a more advanced model, as described later in this book. In other cases it is sufficient merely to refine the conventional Archie interpretation. In this chapter the ways the Archie model may be refined will be discussed. 3.1 NET SAND DEFINITION If core data have been acquired, it is essential that the petrophysicist pay a visit to the core shed at the earliest opportunity to inspect the slabbed core. This will provide a check that there are not anomalous zones that have been wrongly allocated to reservoir or nonreservoir status in the interpretation. Where reservoir can be easily identified, one should make measurements of the core to ascertain the exact net sand footage that can be checked against the calculations made on the logs. 49
    • 50 Well Logging and Formation Evaluation In order to match the net sand footage calculated from the logs with that seen on the core, the shale volume (Vsh) cutoff may be varied. Core photographs will be taken under both normal and UV light, which can also assist in the determination of net reservoir. Once the conventional core analyses have been completed, one will have regular measurements of core porosity, grain density, and permeability. If measurements at overburden conditions have been performed, then the conversion factors to convert porosity and permeability to in-situ conditions should be established. If they are not available, one should assume values based on regional data until special core analyses (SCALs) are completed. In-situ porosity vs. logarithm of permeability should be plotted, if necessary dividing the data according to facies and/or formation such that a single line can be fitted to the data with reasonable accuracy. This yields the so-called poroperm relationship, which is usually of the form (in millidarcies [md], porosity as fraction): k = 10 Ÿ (ka + kb * f) (3.1.1) where k = permeability of the reservoir. Typical values of ka and kb are -2 and 20, respectively. Using the Vsh cutoff chosen, it should be the case that the net sands should not contain porosities much below a level corresponding to 1 md permeability in oil zones and 0.1 md in gas zones. If this is not true, then it may be necessary to apply an additional porosity cutoff to exclude tight zones, which are not picked up purely by a Vsh cutoff. Where core data are not available, it is sometimes helpful to plot the gamma ray (GR) vs. the density log to help to establish the best point to discriminate net from non-net from the GR log. Typically the plot will show a behavior as shown in Figure 3.1.1. As shale becomes dispersed in the pore space (increasing GR), the density will rise until the point at which the pore space available for free fluids becomes zero. Beyond this point, the amount of shale may still increase until the formation becomes 100% shale, but the density will change only slightly (depending on variation in density between quartz and shale). The correct cutoff point is therefore the point at which the gradient changes, corresponding to zero effective porosity. If radioactive minerals are present in the sands, deriving Vsh from the GR alone will not be appropriate. In such formations it is recommended to use purely a porosity cutoff. In the case of thinly laminated sands, it is
    • 51 Full Interpretation 3 2.9 Density (g/cc) 2.8 2.7 log data model GRsa GRsh cutoff point 2.6 2.5 2.4 2.3 2.2 2.1 2 0 50 100 150 GR (API) Figure 3.1.1 Determining Reservoir Cutoffs Using a GR-Density Crossplot possible that the entire formation interval will be designated as nonreservoir using a Vsh or porosity cutoff. In this situation it is recommended to not apply any cutoffs whatsoever. The Archie approach will no longer be appropriate, and advanced techniques should be adopted. 3.2 POROSITY CALCULATION In most cases the density porosity, with an appropriate choice of fluid density, is still recommended. However, a calibration against the conventional core analysis, corrected to in-situ conditions, should be made. The core data should be depth-shifted to match the logs and plotted together with the calculated porosity. A histogram should be made of the core grain density measurements to determine the appropriate value to use in the sands. Note that it is not appropriate to include plugs taken in clearly nonreservoir sections within the analysis. The histogram should provide the mean grain density, as well as give an indication of the likely possible spread of values that could be encountered. The next step is to make a crossplot of the log density against the insitu core porosity values, as shown in Figure 3.2.1: When the core porosity is zero, the density should be equivalent to the core grain density. Also, when the core porosity is unity, the density should be equivalent to the fluid density. The normal procedure is to fix the line
    • 52 core porosity Well Logging and Formation Evaluation log density (g/cc) Figure 3.2.1 Core Calibration of Porosity through the data so that the core grain density is honored, and then to extrapolate the line to the point at which core porosity is unity to determine the appropriate fluid density. Note that this has to be done separately in any gas, oil, and water legs. In theory the fluid densities thus derived should be close to those assumed during the quicklook analyses. Differences might occur due to: 1. 2. 3. 4. Slight miscalibration of the density log Effect of certain mud chemicals (e.g., barite) on the density log Invasion being less or more than previously assumed Problems with the core plug measurements or conversion to in-situ Whatever the reason for the apparent fluid densities being what they are, the combination of the assumed grain and fluid densities, when applied to the density log, will at least ensure that the log porosities match the core densities. Where the fluid densities are anomalous, one would probably want to use them in only the current well, and possibly only over the cored interval. If, however, the densities agree with the values
    • Full Interpretation 53 expected, then they may also be applied with confidence in other wells drilled using similar drilling parameters. 3.3 ARCHIE SATURATION SCAL data measurements of cementation (m) and saturation (n) exponents should be incorporated into the Archie model. In m measurements, the plugs will have been flushed with a brine of an equivalent salinity to that expected in the reservoir and the resistivity measured. By plotting the logarithm of formation factor, given by log(F) = log(Ro /Rw), against log(porosity), according to Archie: log( F ) = - m * log(f) (3.3.1) Therefore, the gradient of the line gives m. Note that the higher the m value used, the higher the water saturations, Sw, that will be calculated, and vica versa. In n measurements, the plugs will have been flushed with brine, then desaturated (either with air or kerosene) to yield measurements of true resistivity, Rt, vs Sw. By plotting the logarithm of the resistivity index, given by log(I) = log(Rt /Ro), against log(Sw), according to Archie: log( I ) = - n * log( Sw ). (3.3.2) Therefore, the gradient of the line gives n. Note that the higher the n value used, the higher the Sw that will be calculated, and vica versa. Values of n that are anomalously high (above 2.5) may be indicative of a mixed or oil-wet system and require further investigation. Low values of n correspond to good-quality water-wet permeable rock. Having set m and n, there is no longer complete freedom to choose Rw if one is required to calculate Sw = 100% in known water sands. If formation-water salinity is well known from produced water samples, one is sometimes faced with a dilemma of whether to honor m or Rw. In many cases, the true cause of this discrepancy is actually an error in the porosity calculation. However, where the porosities are robust, one has to make a choice whether to change m or Rw. It is always worth looking again closely at the cementation-exponent measurements to see how much scatter in the data there is and whether or not the m value chosen is really reliable. If the measurements do not come from the water leg at all, it is possible that diagenetic effects in the reservoir mean that values from the
    • 54 Well Logging and Formation Evaluation oil leg are not representative. However, it may also be the case that the Rw value is not robust for reasons highlighted in Chapter 2 (Section 2.5). With respect to the value of Rt to be used, one needs to decide whether invasion or shoulder-bed effects are significantly affecting the deepest reading resistivity tool. For a well drilled with OBM (oil-based mud) that encounters thick sands, I would recommend simply using the deep resistivity as it is. Where there are significant effects of invasion or shoulders, then generally I would always recommend going with a saturation/height approach in favor of Archie. If it is decided to still try to correct the resistivity for such effects, then the contractor’s chart books may be used for making the appropriate corrections, or computer-based algorithms may be applied. Remember that such corrections apply equally in the water leg if one is using a Pickett plot to determine m and Rw. 3.4 PERMEABILITY For the final evaluation, a permeability log, as well as zonal averages, will usually be required for input to the static and dynamic models. Using the poroperm relationship described in Section 3.1, it is relatively simple to derive a permeability log using the porosity log. However, once the log has been derived, it is important to scrutinize it for any intervals for which the permeability goes to an anomalously high value. Most sandstones do not exceed about 1500 md, although top-quality sands with porosities above 35% may have permeabilities up to about 4000 md. If necessary, apply a cutoff to cap the permeability at a level that is supportable by the core data. In the nonreservoir sections, the permeability should usually be set to a very low value (e.g., 0.001 md). Permeabilities calculated should be roughly in line with those calculated from other sources, such as a formation pressure tool, NMR (nuclear magnetic resonance), or production tests. For making zonal averages of the permeability, it should be noted that three types of average are possible: arithmetic, geometric, and harmonic. The arithmetic average is given by: karith =  ki * hi  hi (3.4.1) Hence, if, say, the zone were 50 ft, comprising 100 k values at 0.5-ft spacing, the average would simply be the sum of the 100 values divided by 100. This average is appropriate to use if the flow in the reservoir is
    • Full Interpretation 55 in the direction of the bedding plane. Small, impermeable streaks will have only very little effect on the average. The geometric average is given by: kgeom = exp( log(ki ) * hi  hi ) (3.4.2) In effect, one takes the average of the logarithms of the individual k’s, then takes the exponential at the end. This average is appropriate to use if the flow in the reservoir is partially in the direction of the bedding plane and partly normal to it. Impermeable streaks will have some influence but not completely kill off the zonal average. The harmonic average is given by: kharm = 1 ( (hi ki )  hi ) (3.4.3) In effect, one takes the average of the inverse of the individual k’s, then inverts the result at the end. This average is appropriate to use if the flow in the reservoir is normal to the direction of the bedding plane. Impermeable streaks will completely dominate the zonal average. Depending on which method is used, the petrophysicist can get widely different results. Typically the arithmetic average will be at least 10 times higher than the harmonic, with the geometric lying somewhere in between. Note that in horizontal wells there is an additional effect due to the fact that kv / kh on the microscopic scale is usually less than 1. The effect of this may be estimated as follows. Let a = kv / kh, where kv = permeability of the vertical well and kh = that of the horizontal. It may be shown that the average permeability (kav) seen by the wellbore, which will be partially influenced by kv and partly by kh, is given by: 2p kav = (kh 2 * p ) * Ú sqrt (cos 2 (q ) + a * sin 2 (q ))dq = kh * (1 + a ) 2 0 The result, for various values of a, is shown in Figure 3.4.1. The parameter of kv / kh will generally be assumed over an entire reservoir within a dynamic model. Typical values are between 0.1 and 0.3. From Figure 3.4.1, it may be seen that the permeabilities, as determined from a poroperm relationship, need to be adjusted in a horizontal well, even if the formation appears homogeneous throughout. When giving zonal averages, it is usual to also include the product k * h, where h is the thickness of the zone, since it is this which can be
    • 56 khor /karith Well Logging and Formation Evaluation kv /kh Figure 3.4.1 Effect on Average Permeability of kv / kh < 1 in horizontal well related to the flow generated in a production test. Where log-derived values of k * h are compared with production tests, it is often the case that the result derived from the arithmetic average will be higher than that seen in the production test. Reasons for this are: • • • • Not all the perforated zone contributes to the flow, and the actual h is smaller than that assumed in the petrophysical calculation. Some of the flow is not parallel to the bedding plane. Formation damage (called “skin”) has occurred between the openhole logging and the testing operation. While the test analysis seeks to identify this as a separate term from k * h, it may still be partly incorporated into the calculated kh quoted. Relative permeability effects, such as gas blocking, may be occurring, making the lab-calibrated in-situ brine permeability inappropriate. Differences between the log-derived and test permeabilities are a fact of life in real reservoirs and do not necessarily invalidate the log-derived
    • Full Interpretation 57 permeabilities that find their way into the static and dynamic models. What happens in practice is that during the history-matching process in the simulator, the permeabilities may be adjusted either globally or near certain wells in order to make the predicted flow rates match the production data. Exercise 3.1. Full Evaluation of the Test1 Well Use the data in Appendix 2 to: 1. Revise your net sand discrimination criteria if necessary 2. Calibrate the density log against core porosity. Assume a net effective stress of 2000 psi. 3. Derive a poroperm relationship 4. Derive revised values of m and n to use 5. Recalculate the sums and averages and additionally calculate average permeabilities (using arithmetic, geometric, and harmonic averaging) and k * h 6. Calculate the total equivalent hydraulic conductivity (EHC) (thickness * porosity * Sh) and compare with your quicklook results.
    • C H A P T E R 4 SATURATION/HEIGHT ANALYSIS The reason for putting this chapter before other advanced interpretation techniques is that I believe it is of primary importance in correctly defining the STOIIP (stock tank oil initially in place) or GIIP (gas initially in place) of a field. Indeed, because of the way that dynamic reservoir models are constructed in many fields in practice, it completely supersedes any exotic models constructed by the petrophysicist for calculating saturations. Many times in my career I have seen petrophysical departments working in isolation constructing fabulously complicated models to calculate saturations. But when you ask the geologist what saturations have gone into the static model, he will tell you that he is using a constant value unrelated to the zonal weighted averages. The reservoir engineer may be using one Pc /Sw table in the simulator based on just one air/mercury capillary pressure measurement that he felt was representative of the reservoir in general. I believe perhaps the most important role the petrophysicist has in a petroleum engineering department concerns ensuring that the saturation/height function being used in the static and dynamic models represent the best possible combination of core and log data, combined with sound petrophysical judgment. In my view, such a function should have both porosity and permeability as input variables, together with height (which may be directly related to Pc). There are dozens of different functions that have been used to describe capillary behavior in rocks. I have used many of these over my career, but I have found the Leverett J function to be the most broadly applicable. I therefore propose to describe how such a function may be constructed for a reservoir, using both core and log data. This function may be stated thus: Sw = Swirr + a * J b (4.1) 59
    • 60 Well Logging and Formation Evaluation where J = Pc * [ (k f) ] (s cos(q )) (4.2) Pc = ( rho w - rho h ) * h * 3.281 * 0.433 (4.3) Swirr = irreducible water saturation rhow = formation water density, in g/cc rhoh = hydrocarbon density, in g/cc Pc = capillary pressure, in psi k = permeability, in md f = porosity (as fraction) s = interfacial tension between the hydrocarbon and water, in dynes/cm2 q = contact angle between the hydrocarbon and water, in degrees h = height above the free water level (FWL), in m the constants a and b are to be fitted to the data. Note that units are not particularly important, as long as they are used consistently throughout. If, say, pressures are used in bars instead of psi, the effect will be for a and b to be modified, but the results will be the same. 4.1 CORE CAPILLARY PRESSURE ANALYSIS The results of a SCAL program of Pc measurements will usually be presented in the form of Table 4.1.1. Note that figures in the body of the table represent Sw values. Note also that these measurements will have been performed by one of a number of methods, none of which use actual formation fluids. Use the following steps to generate the average J function. Let: Swr = Sw - Swirr . (4.1.1) 1. Convert the table above into a table of J vs. Swr. Set Swirr equal to 0.01 below the lowest water saturation seen anywhere in the reservoir in cores or logs. In order to derive J, use the values for k and f in the table. For the interfacial tension and contact angle, use the data in Table 4.1.2 depending on the type of measurement used: 2. Plot Log(J) vs. Log(Swr). The intercept and gradient should give you the constants a and b (Figure 4.1.1).
    • 61 Saturation/Height Analysis Table 4.1.1 Example of core-derived drainage capillary pressure curves Pc (psi) K f 0.078 0.084 0.100 0.096 0.107 0.108 0.123 0.125 0.126 3.000 10.000 25.000 50.000 125.000 200.000 0.347 0.992 2.828 8.782 18.350 11.609 42.215 60.976 157.569 0.850 0.839 0.763 0.659 0.548 0.651 0.457 0.566 0.377 0.783 0.745 0.488 0.353 0.304 0.325 0.270 0.348 0.225 0.614 0.525 0.371 0.261 0.218 0.237 0.180 0.258 0.147 0.491 0.386 0.281 0.216 0.170 0.198 0.158 0.241 0.127 0.386 0.295 0.233 0.201 0.164 0.191 0.155 0.204 0.121 0.352 0.269 0.210 0.200 0.165 0.193 0.155 0.200 0.120 Table 4.1.2 Typical values of interfacial tension and contact angle for reservoir conditions s (mN/m) Air/mercury Air/brine Kerosene/brine Air/kerosene Cos(q) 480 72 48 24 0.765 1.0 0.866 1.0 Having defined Swirr, a, and b, you basically have all the information you need to construct a saturation/height function for any given porosity and permeability. It is often convenient to create a special poroperm-type relationship using the plug k and f values (Figure 4.1.2). Such a relationship should have the form: k = 10 ( ka+kb*f ). (4.1.2) When constructing J in the reservoir, you will need to use the s and q values corresponding to your reservoir conditions. Since these are generally not known, it is recommended to refer to Table 4.1.3. Having defined such a relationship, it is possible to produce a set of generic saturation/height functions for a range of porosities typically encountered in the reservoir, as shown in Figure 4.1.3:
    • 62 Well Logging and Formation Evaluation 1 1 10 100 Swr 1 2 3 4 5 6 7 8 9 model 0.1 J perm (md) Figure 4.1.1 Fitting a J function to Core Data por Figure 4.1.2 Fitting a Poroperm Relationship to the Capillary Pressure Plugs
    • 63 Saturation/Height Analysis Height (m) 120 100 100 80 80 60 60 40 40 20 Pc (psi) 120 20 0 0.0 0.13 0.06 0.08 0.10 0.11 0 0.2 0.4 0.6 0.8 1.0 Sw Figure 4.1.3 Generic Saturation/Height Curves from the J Function Table 4.1.3 Typical values of interfacial tension and contact angle for reservoir conditions s (dynes/cm2) Gas/water Oil/water Cos(q) 50 30 1.0 0.866 These curves, in the form of Pc /Sw tables for particular porosity classes, may be handed directly to the reservoir engineer for inclusion in a simulator. If a geologist wants an “average” value of Sw to use for a static model, the curves should be averaged over a realistic column height for a particular porosity class. Better still is to obtain an area/height table from the geologist and to further weight the curves according to their relative areas at a given height. Since most reservoirs are wider at the base than at the crest, this results in a higher average water saturation.
    • 64 Well Logging and Formation Evaluation If you have a gas column above an oil column in the reservoir, generate two set of tables, one for an oil/water system and one for a gas/water system using the appropriate s, q, rhoh, and rhow values. Exercise 4.1. Core-Derived J Function 1. Based on the core capillary pressure measurements presented in Appendix 2, derive a J function. 2. Recalculate the sums and averages using the J function derived from these core data. 4.2 LOG-DERIVED FUNCTIONS The methodology applied to core data can also be applied to the logs to derive an independent J function, provided that the position of the FWL is known. This function is useful for comparison with the core-derived function and may be used if no core data are available. A secondary benefit is that it is possible during the fitting procedure to eliminate thin bed and invasion effects and provide a means of generating a high-resolution saturation log that depends on only the density log. In order to fit a J function, follow these steps: 1. Use the depth of the FWL and the true vertical depth subsea (TVDss) to generate a curve for the height above FWL. 2. Use equation 4.3 to derive a curve for Pc. 3. Use equation 4.2 to derive a curve for J. It is assumed that you already have some kind of poroperm relationship that can be used to relate k to the porosity. 4. Use equation 4.4 to derive a curve for Swr. Set Swirr to be 0.01 less than the lowest Sw seen on the logs. 5. Make a crossplot of Log(J) vs. Log(Swr). You will find a cloud of points. Note that shoulder-bed effects and invasion of water-based mud will always pull points to higher Swr values compared with points originating from thick beds and for which less invasion is occurring. Therefore, by fitting the constants a and b so that the model follows the leading edge of the cloud of points, you will effectively be correcting for thin bed and invasion effects.
    • Saturation/Height Analysis 65 Having derived a and b, you are now in the same position as you were at the end of the Pc-curve averaging exercise. Generic saturation/height functions can be derived in an identical manner as before. The curves should be compared with those derived from Pc, or “cap,” curves and an attempt made to explain any differences. Differences may arise from the following: • • • • • The core measurements, particularly if performed using air/mercury, may be unrepresentative of the reservoir as seen by the logs. There may be a discrepancy in the permeability transform used for the core plugs compared with that used on the logs. The assumed position of the FWL may be wrong. The logs may be influenced by other effects, making either the porosities or the saturations calculated erroneous. Depletion may have occurred prior to logging. In cases where the discrepancies cannot be explained, it may be considered reasonable to take the more pessimistic set of functions as a “low” and the optimistic ones as a “high” and construct a baseline lying halfway between. Having decided on a baseline function, the logs should be reevaluated using the J function in reverse to recalculate saturations. Considering that the function will use only the curves for height above FWL and porosity/k as input (all derived from the density log), the results may prove surprising. Having applied this technique on more than 20 reservoirs, I have always been amazed at how reasonable the results look, and often how well they compare with the resistivity-derived Sw values, except in the thin beds, where resistivity-derived Sw is usually too high. The density log will typically have a vertical resolution of 1 ft, far superior to the deep induction or laterolog. The resulting increase in average oil or gas saturation will typically be on the order of 20%. I believe that a properly calibrated Leverett J function, reconciling reliable log saturations with core data, represents the best possible way to calculate saturations in a reservoir and to transfer this information into static and dynamic models. It should also be recognized that where core data are reliable but the position of the FWL is unknown, the function may be used to predict the depth of the FWL from the log saturations. In general I have not had much success in applying this technique, but it may merit further investigation.
    • 66 Well Logging and Formation Evaluation Exercise 4.2. Log-Derived J Function 1. Use your results from Exercises 2.1 and 3.1 to derive a J function based on the log data. 2. Recalculate the sums and averages using the J function derived from the logs. 3. Do you recommend using the function derived from the core or log for future STOIIP determination? What are your reasons?
    • C H A P T E R 5 ADVANCED LOG INTERPRETATION TECHNIQUES 5.1 SHALY SAND ANALYSIS Shales can cause complications for the petrophysicist because they are generally conductive and may therefore mask the high resistance characteristic of hydrocarbons. Clay crystals attract water that is adsorbed onto the surface, as well as cations (e.g., sodium) that are themselves surrounded by hydration water. This gives rise to an excess conductivity compared with rock, in which clay crystals are not present and this space might otherwise be filled with hydrocarbon. Using Archie’s equation in shaly sands results in values of water saturations, Sw, that are too high, and may lead to potentially hydrocarbon bearing zones being missed. Many equations have been proposed in the past for accounting for the excess conductivity resulting from dispersed clays in the formation, which can have the effect of suppressing the resistivity and making Sw calculated using Archie too pessimistic. While these equations will be given, I propose to work only one method through in detail, namely a modification to the Waxman-Smits approach. I have successfully used this method in a number of fields, and it has the advantage of not necessarily relying on additional core analyses for calibration (although these data may be included in the model). Waxman-Smit’s equation may be stated as follows: Sw n* = [( Rt Rw ) * f m* * (1 + Rw BQv Sw )] (5.1.1) where B is a constant related to temperature, and Qv = cation exchange capacity per unit pore volume. Here m* and n* have a similar definition 67
    • 68 Well Logging and Formation Evaluation as with Archie but are derived in a different way from core data. Temperature determines the excess conductivity (in ohms) resulting for the clay per unit Qv (in cc). An equation that may be used to relate B to formation temperature and water resistivity, Rw, as published by Thomas, is as follows: 1 B = ( -1.28 + 0.255 * T - 0.0004059 * T 2 ) (1 + (0.04 * T - 0.27) * Rw.23 ) (5.1.2) where T is measured in degrees Celsius. Qv (in meq/unit pore volume in cc) is related to the cation exchange capacity (CEC) (in meq/100 g) of the clay as measured in a laboratory. Qv may be derived from the CEC using: Qv = CEC *density (100 * f) (5.1.3) CEC can be measured by chemical titration of crushed core samples. It is dependent on the type of clay. Typical values for clay types are shown in Table 5.1.1. It has to be said that core-derived measurements based on crushed samples are probably unrepresentative, since the crushing process will expose many more cation exchange types than will be available in the formation. Moreover, I have never been able to relate Qv to any log-derived parameter (e.g., porosity, Vsh) with any success. A further uncertainty relates to the factor B, which is typically derived using a “standard” correlation that may or not be applicable. A far better approach is to derive the combined factor BQv from logs in a known waterbearing sand. I will now present a useful method of doing this. First of all, the assumption is made that BQv obeys an equation of the form: BQv = (fc - f) (C * f) . (5.1.4) Table 5.1.1 Typical Properties of Clays Clay Kaolinite Illite Montmorillonite Chlorite CEC (meq/100 g) 3–15 10–40 80–150 1–30 Grain Density (g/cc) Hydrogen Index 2.64 2.77 2.62 3.0 0.37 0.09 0.12 0.32
    • Advanced Log Interpretation Techniques 69 In effect this equation makes the assumption that there is a “clean porosity” (fc) and that reduction in the measured porosity is as a result of dispersed clay. The excess conductivity BQv is assumed to be related to this proportional porosity reduction via the constant C. If equation 5.1.3 is inserted into equation 5.1.1 and Sw is set to 1 (for water sands), the equation may be rearranged as: Cwa = f - m* Rt = 1 Rw + (fc C ) * (1 f - 1 fc ) (5.1.5) Hence if Cwa is plotted against 1/f for water sands, the points should fall on a line such that: • • • The Cwa value at the start of the data cloud represents 1/Rw. The (1 / f) value at the start of the data cloud represents 1/fc. The points should fall on a gradient equal to (fc /C ). Cwa Note that if there are no shaliness effects, the points should simply create a horizontal flatline from which 1/Rw can be read off, with the factor C becoming infinite. If the data fail to fall on a single trend, as above, then the method may be deemed to be inappropriate. An example of some data plotted in this way is shown in Figure 5.1.1. 1/por Figure 5.1.1 Cwa / (1 / f) Plot
    • 70 Well Logging and Formation Evaluation This plot was made assuming an m* value of 2.0. From the plot it was possible to determine: Rw = 0.05 ohmm; fc = 0.13; and C = 0.01. Using an assumed value of n*, it is then possible to calculate Waxman-Smits saturations in the hydrocarbon leg. If SCAL (special core analysis) data are available, it is possible to derive m* and n* from the experiments as follows. In a conventional cementation-exponent (m) measurement, the formation factor F is plotted against f on a log-log scale. In Archie’s model, the following would be true: F = ( Ro Rw ) = f - m (5.1.6) where Ro is the resistivity of the 100% water-saturated rock. Hence log(F) = -m*log(f) and the gradient of the line yields m. Since for the Waxman-Smits equation it is clearly not the case that F = f-m *, a correction must be made. Let: F* = (1 + Rw BQv ) * F = f - m* (5.1.7) Now, if F* is plotted against f on a log-log scale, it is indeed the case that the gradient yields m*. Having derived m*, it should then be used to rederive Cwa from Rt and f. This will then lead to revised values of Rw, fc, and C. This may in turn lead to a revised value of m*. Usually a couple of iterations are sufficient to get m* to converge to a value that fits both the Cwa vs. 1/f plot and the F* vs. f plot. Since F* will exceed F by a larger amount at low porosities than at high porosities, m* will always be greater than m. Typically, if an m value of 2.0 is measured, the value of m* will be around 2.2. A similar procedure is followed for n*. Archie’s model assumes that: I = ( Rt Ro ) = Sw n (5.1.8) Hence, if log(I) is plotted against log(Sw), the gradient should yield n. For Waxman-Smits, it is necessary to derive I*, which is given by I * = (1 + Rw BQv Sw ) * I (1 + Rw BQv ) = Sw n *. (5.1.9) Plotting log(I*) versus log(Sw) yields the corrected saturation exponent n*. As with m*, n* will be found to be lower than n, typically by about 0.2.
    • 71 Advanced Log Interpretation Techniques Similar to the Pickett plot made with Archie, the value of the hydrocarbon saturations is not very sensitive to the value of m*, provided that a water sand is used to calibrate the value of Rw. Note that the value of Rw derived may not correspond to the salinity expected from production tests. This value also includes the effect of the clay-bound water, which may be fresher than the free water and will not flow during production. Hence, it is typically found that Rw appears higher than expected. The effect of using Waxman-Smits will usually be large only for relatively high values of Rw. This is because the factor RwBQv becomes small compared with unity if Rw is small (saline environments). In this situation the calculated Sw will differ only very slightly from that calculated using Archie’s model. Note that when the equation is applied, a computational complication arises from the fact that Sw appears on both sides of the equation. This can be easily overcome as follows. Initially assume that the value of Sw in the right-hand side of equation 5.1.1 is unity. Calculate Sw and reinsert the new value of Sw into the right-hand side of the equation. Continue in this way until the Sw on the left-hand side ceases to change beyond 0.001 with successive iterations. Typically, five or so iterations are sufficient. Another way to apply Waxman-Smits method is by the so-called normalized Qv method, as proposed by Istvan Juhasz. Readers are recommended to read the relevant paper from the Society of Petrophysicists and Well Log Analysts that covers this method in detail (see references). A condensed version will be given here. Juhasz shows that the WaxmanSmits equation may be rearranged in the form: 1 n* Sw = [(f - m* Rt ) * ( Sw * Rwsh * Rw ) (Qvn * Rw + ( Sw - Qvn ) * Rwsh )] (5.1.10) where Qvn = Vsh * fsh f (5.1.11) 1 Rwsh = f - m* Rsh (5.1.12) Rsh = resistivity of the shale fsh = porosity of the shale. The parameter m* may be determined from plotting log(Rt) vs. log(f) in water-bearing shaly zones (not clean zones), since the slope of the line is equivalent to m*. The parameters Rwsh and Rw may be determined from plotting Cwa vs. Qvn, since the intercept of the points for Qvn = 0 on the
    • 72 Well Logging and Formation Evaluation Cwa axis gives 1/Rw, and the intercept for Qvn = 1.0 gives Rwsh. Other equations that are commonly used are: Dual-water model: 2 1 Rt = Sw ( F * Rw ) + BQv * Sw F (5.1.13) where F = f- m . (5.1.14) Simandoux: n Sw = A * Rw * f - m * {1 Rt - Vsh * Sw Rsh } (5.1.15) where Rsh is the resistivity of the shale. Indonesia equation: ( Sw = Rt( -1 n ) * {Vsh10 Vsh 2) Rsh + f( m 2 ) ( A * Rw ) } ( -2 n ) (5.1.16) Since Waxman-Smits fulfills all the criteria I require from a shaly sand equation (i.e., it introduces a clay conductivity element that is related to the amount of clay as determined by the logs), I have only rarely used other equations during my career. As stated in Chapter 3, I would always prefer to derive saturations for STOIIP (stock tank oil initially in place) and GIIP (gas initially in place) using a saturation/height function calibrated against good-quality core measurements. Exercise 5.1. Shaly Sand Analysis 1. Using data from the water leg in the test1 well, make a relationship of BQv to porosity. Also derive Rw from the plot. 2. Using the core data in Appendix 2, calculate m* and n*. 3. Calculate saturations using the Waxman-Smits equation and make a new table of sums and averages. 4. How do the Waxman-Smits saturations compare with those derived using Archie and those from the core-derived J function?
    • Advanced Log Interpretation Techniques 73 5.2 CARBONATES Most of the analyses presented in earlier chapters work equally well with carbonate formations. However, the grain density used for deriving porosity from the density log needs to be set to a different value. For limestones the most common value used is 2.71 g/cc, although a direct measurement on the core is preferred, if available. Dolomite and anhydrite have higher matrix densities (see Appendix 6). In many fields with goodquality carbonate reservoirs, evaluation is simpler than with sandstone because the shaly sand problem does not occur and beds tend to be thicker. In general, it will be found that Archie’s model works very well. However, during my career I have also worked on some extremely tight gas- and oil-bearing carbonate reservoirs where normal techniques did not give good results. I will therefore discuss these. Typical characteristics of low-permeability (<1 md) carbonate reservoirs are: • • • Porosity may still be appreciable but be in the form of isolated pockets or vugs. This is often seen in chalk reservoirs. Entry heights and transition zones may be extremely long (up to 100 m). Matrix permeability may be extremely low, but the well may flow due to the presence of natural (or mechanically induced) fractures in the formation. Due to the fact that these fractures may cause significant losses during drilling, the well may need to produce back a lot of drilling fluid before formation hydrocarbons start to flow. In such a field, the development strategy will often be to drill horizontal wells perpendicular to the natural fracture orientation, thereby optimizing the productivity of the wells. The STOIIP or GIIP will be stored mainly in the matrix but flow in the well via the fractures. Hence, if there is not an extensive fracture system that provides a conduit for the matrix to flow into, the wells will either water out or die very quickly. Clearly the ability to detect the presence and orientation of fractures is extremely important. During drilling it may be useful to monitor the extent of losses, and depths at which they start to occur, which may provide valuable information as to the presence of fractures. Needless to say, the measures adopted by drilling to cure these losses need to be examined to ensure that any fracture permeability is not permanently impaired. Conventional logs that may give indication of fractures are: • • Sonic log (cycle skipping occurrence) Microresistivity tool (erratic behavior as the pad crosses a fracture)
    • 74 • • Well Logging and Formation Evaluation GR (spikes occurring where fractures have become cemented up with radioactive minerals) Caliper (borehole will tend to become elliptical, with the major axis perpendicular to the tectonic stress direction) In order to properly characterize the types and orientation of fractures, it is necessary to use imaging tools. Resistivity tools are preferred because it is easier to differentiate open fractures filled with fluid from cemented fractures. However, ultrasonic-based tools can also be used and are the only option in oil-based mud (OBM). Note that naturally occurring fractures will tend to be oriented in the direction of maximum horizontal stress in the field. Particularly in areas close to major fault systems, the difference between the stresses in different directions may be very large and conducive to fracturing. Cores may also prove invaluable in characterizing any fractures that may be present, although care has to be taken to exclude drilling-induced fractures. It is also necessary that an orientation tool be run with the coring assembly. Once the fracture system has been analyzed, it is useful to derive a fracture density curve that may be included with the other logs. Such a curve may be correlated with the horizontal permeability kh derived during a well test and used to predict the producibility of future well penetrations. Carbonate reservoirs, unlike clastic reservoirs, may well be amenable to HCl acidization treatments either with or without mechanical fracturing (“fraccing”) of the reservoir. However, such treatments can affect only the region around the wellbore and will not compensate for poor permeability and lack of fractures over the wider extent of the field. Pressure testing/sampling of tight carbonate reservoirs using a conventional probe is nearly always unsuccessful. In order to have any hope of success, one would need to use a packer-type of tool, and even then the success rate is typically low. In chalk reservoirs, compaction may well be an issue during field life. While this has the advantage of providing an additional pressure support mechanism, extensive studies will be required during the design phase of any installations, particularly offshore. 5.3 MULTIMINERAL/STATISTICAL MODELS As stated in Chapter 2, my preferred method of calculating porosity is the density log using the appropriate matrix and fluid densities. This approach can go badly wrong if heavier minerals are also present in the
    • Advanced Log Interpretation Techniques 75 formation in variable amounts. I have worked in some fields having varying amounts of limestone, marl, anhydrite, dolomite, siderite, pyrite, quartz, and clays where a conventional approach using deterministic equations is not reliable. In such a situation, the best approach is to adopt a multimineral/statistical model. The basic way programs using this technique operate is as follows: 1. The various minerals and fluids to be included in the model are determined. 2. The response of each of these minerals/fluids for a variety of parameters as measured by logging tools is specified by the user. 3. The program finds the combination of mineral/fluid volume fractions that most closely matches the observed log responses, such as to a variety of criteria and constraints specified by the user, such as the: ᭿ relative importance (weighting) of various tools, ᭿ measurement error for each tool, ᭿ relative saturations of fluids in the invaded zone as opposed to the virgin zone, ᭿ relative amounts of various minerals, and ᭿ resistivity response relative to fluid saturations (e.g., Archie, dualwater, Waxman-Smits, etc.). Needless to say, it is not possible to have a greater number of minerals/fluids than the number of tool response equations (although the fact that the sum of all the volume fractions must equal 1 effectively provides an extra equation). Overall, this sounds like a very rigorous approach to a conventional sort of deterministic evaluation, which would be preferred in all cases. Reasons why it is not are as follows: 1. The fact that the program is capable of calculating back the correct log responses does not mean that the results are necessarily correct. If a bad choice of minerals/fluids is made or their properties are incorrect, a solution may be found that is completely wrong. 2. The program does not have any depth-dependent reasoning capability. Hence, one may frequently find “a bit of everything everywhere,” with gas below oil and oil below water and minerals popping up all over the place, often more in response to the hole quality than anything else. 3. Many of the tool responses for minerals in the formation are not known accurately and recourse is made to standard tables of typical minerals.
    • 76 Well Logging and Formation Evaluation It is found that changing the value of one parameter can have a drastic effect on the resulting volume fraction of that mineral. 4. Porosities derived by the program will typically come about from a combination of the density, neutron, and sonic responses, plus whatever assumption is made about the relative saturations in the invaded zone as opposed to the virgin zone. I am dubious about how correct such a porosity really is. 5. Unlike with a deterministic approach, where the resulting uncertainty in porosity and saturation may be directly traced to uncertainty in the input parameters, statistical programs represent a “black box” approach, in which there is no clear audit trail between the input and the output. 6. The programs are very sensitive to log quality and noise on the log traces. Where even one log is reading wrong, the volume fractions will be affected and the output may be completely unreliable. Overall, my impression is that there are some cases in which statistical models offer real advantages over conventional interpretations. A good example is in a sandstone reservoir having variable amounts of siderite or pyrite. The program is also useful in a normal sandstone reservoir where there are limestone stringers intermittently present. In situations where the mineralogy is not well known or the logs are of poor quality, I am extremely dubious about the quantitative correctness of the output. Even when neither of these situations arises, it is still my experience that it is necessary to make dozens of runs, investigating the effects of minor changes to the input parameters, before a solution can be produced in which one can have any confidence. All the main logging contractors are able to offer software for statistical analyses, which can be run either by the contractor or in-house by the oil company. The values for typical parameters for various minerals are usually built into the software as default values, so will not be repeated here. 5.4 NUCLEAR MAGNETIC RESONANCE LOGGING When first introduced in the late 1980s, NMR logging attracted a lot of interest because it was a whole new type of measurement and offered the possibility of direct measurement of porosity and the differentiation of fluid types and the relative contributions arising from clay-bound water from free water. In this respect it offered to solve one of the problems occasionally confronting petrophysicists, namely, low-resistivity pay.
    • Advanced Log Interpretation Techniques 77 The basic principle by which the tool operates is as follows. The tool is assumed to respond only to hydrogen nuclei (in water, oil, and gas) in the porespace. The hydrogen nuclei (which are just protons) in the porefluids have a spin and magnetic moment that may be affected by an external magnetic field. In the absence of an atomic field, these moments are aligned randomly. When an external field (B0) is applied, a process occurs whereby the orientation of the nuclei changes so that a proportion of them align in the direction of the applied field H. The reason they do not all immediately align in the direction of the field is that two adjacent nuclei are in a lower energy state when they are aligned in opposite directions. The nuclei do not immediately align in the direction of H, but their spins precess around B0 at a frequency given by Larmor: w Larmor = gB0 where g is the gyromagnetic ratio (42.58 Mhz/T for hydrogen) and B0 is the strength of the external field, in Tesla. After a time a proportion of the nuclei have “relaxed” to be aligned with B0. The resulting magnetization of the formation is given by Mv, and will vary according to: Mv µ (1 - exp( - t T1 ) where T1 is the longitudinal relaxation time. In the absence of any further fields being applied to the horizontal magnetic components, the individual nuclei will be randomly distributed and sum to zero. Now consider what happens if, after a period denoted by Tw (wait time), a horizontal magnetic field is applied at a frequency equal to the Larmor frequency. The nuclei’s horizontal magnetic moments will start to align themselves in the direction of the horizontal pulse. After a time given by t (the echo time), a pulse is given at 180 degrees to the direction of B0 (called a p pulse). The nuclei start to align themselves in the opposite direction. However, because of differences in their horizontal relaxation times, the magnetic moment building up in the opposite direction will be less than during the first pulse. A third pulse is then applied in the original direction of B0 with correspondingly even less buildup of moment. The process is continued for a finite number of “echoes” until the horizontal signal (which can be detected in the tool’s coils) dies away to zero. The decay of the horizontal signal is called the transverse relaxation, and the magnetization detected (denoted by Mh, ignoring diffusion) will vary according to:
    • 78 Well Logging and Formation Evaluation Mh = M0 ◊ exp( - t T2 ) This pulse scheme is referred to as a CPMG (Carr, Purcell, Meiboom, and Gill) excitation. In practice, not all the fluid in the pores will relax according to the same T2. Those lying close to the pore wall will relax more quickly than those in the center of the pore. This means that there is a series of contributions to Mh, each decaying at a different rate. In addition, because of diffusion of the nuclei within the pores, nuclei that may not initially be close to the wall may move toward the wall during the measurement and relax more quickly. This introduces an extra term into the behavior of Mh, given by: Exp( - t ◊ g 2 ◊ D ◊ G 2 ◊ t 2 3) where D = molecular self-diffusion coefficient and G = gradient of the static magnetic field. Hence the full expression for Mh is given by: Mh = M0 ◊ exp( - t T2 ) ◊ Exp( - t ◊ g 2 ◊ D ◊ G 2 ◊ t 2 3). Because different fluids (oil, gas, water) have different values of D, if measurements are done at different values of t, there is the possibility of differentiating fluid type. This is the basis for what is called time domain analysis (TDA). The influence of the pore wall on T2 is assumed to follow a relation of the form: 1 T2 = 1 T2,bulk + r ◊ S V where T2,bulk = relaxation time of bulk fluid, r = surface relaxivity, and S/V = pore surface-to-volume ratio. For spherical pores of radius r, S/V reduces to r·3/r. Note that the expression for capillary pressure, Pc, is given by: Pc = 2 ◊ (s ◊ cos(q )) r . Hence it can be shown that if T2, bulk >> r·S/V, which is the case close to the pore wall, then: Pc ◊ T2 = (s ◊ cos(q ) 1.5 ◊ r). This is important because it shows how a cutoff between bound and free fluid, made on the basis of Pc, can be translated into a cutoff based
    • Advanced Log Interpretation Techniques 79 on T2 for distinguishing between bound and free fluid. This is the basis for the T2 cutoff commonly used: 33 ms for water-wet sandstones and 100 ms for carbonates. The 33 ms is based on an assumed surface relaxivity of sandstones of 100 mm/s. In fact the relaxivity may vary considerably among different types of rock. Values as low as 14.4 have been reported in the literature. Times as long as 200 ms have also been seen in sandstones drilled with OBM. If the appropriate T2 cutoff is not a constant but is facies dependent, significant problems are caused in determining permeability accurately. In fact, a conventional poroperm approach normally works quite well if a different relationship is used according to facies type. A lot of the potential benefits from NMR are removed if one requires both core T2 measurements for all facies types and a means for determining which facies is being logged. The permeability may also be severely affected if, based on TDA, a gas/oil/water model is being assumed rather than a straight oil/water model. Applying the gas correction may affect the permeabilities by a factor of up to 100. The total porosity of the sample is related to the strength of the initial signal occurring from the tool following the first transverse pulse during a T2 acquisition. Note, however, that for the following reasons this might read low: • • If the wait time Tw (also sometimes denoted as Tr, the recovery time) prior to the CPMG excitation is too short, the transverse field will be reduced. This is referred to as incomplete polarization, to which polarization correction may be applied. The tool is calibrated assuming 100% freshwater in the pores, i.e., the hydrogen index (HI) is 1.0. The HI is influenced by temperature, pressure, and salinity, as well as the fluid type (water, oil, or gas). Because clay-bound water relaxes very fast (T2 of a few ms), a special mode of acquisition is required to measure total porosity. In a normal acquisition mode, the tool will respond to only capillary-bound and free fluids. It should be noted that it is normally assumed that the rock is water wet. This means that any short T2 arrivals are the result of the wetting phase relaxing close to the pore wall. Other fluids, such as gas and water, are assumed to be far from the pore wall, so that one sees only their bulk fluid relaxation times. Any kind of TDA, which exploits the differences in D, exploits this fact. The interpretation of the tool results can be subject to serious errors if this assumption is not true, which can be the case in a well drilled with OBM for
    • 80 Well Logging and Formation Evaluation which surfactants in the OBM filtrate have made the invaded zone oil wet. Even where WBM (water-based mud) has been used, any mixed wettability in the formation will tend to result in anomalous results. The tool will measure a decaying magnetic amplitude vs. time, which depends on the following parameters: • • • • • • • B0 (the static field strength) Tw (or Tr) (the wait time for longitudinal polarization) t (the transverse echo time) T1 of the fluids in the pore space T2 of the fluids in the pore space D of the fluids in the pore space The total porosity Note that the signal will arise from only the part of the formation for which the CPMG pulses correspond to the correct Larmor frequency. Because the fixed magnet is located in the borehole, with the magnetic field decreasing with distance from the borehole, this will define the zone of investigation of the tool. Having measured the transverse signal as a function of time, the next step is to invert these data into the corresponding distribution of T2 values that make up the signal. This would be a straightforward mathematical operation were it not for the presence of noise in the signal. In fact, without some additional form of constraint, at the noise levels typically encountered in the tool it is possible to produce wildly different T2 distributions that can all honor the original decay curve. One constraint that is commonly applied, called regularization, is that the T2 distribution must be smooth. This results in a more stable solution, although there is no particular reason why the T2 spectrum should indeed be smooth. Needless to say, unless the inversion is correct, the results of the tool will be completely useless. This is worth bearing in mind in situations where the tool gives results that cannot be explained in terms of known properties of the lithology based on core data. In practice the T2 spectra are not continuous but divided into “bins” covering different ranges of T2. The maximum value of T2 that can be measured is determined by the time allowed for the signal to be measured. This in turn is related to the logging speed. In some situations, wait times of up to 15 seconds might be needed to capture the full spectra, translating into logging speeds that are very slow (under 100 ft/hour). Output curves common to NMR tools include the following:
    • Advanced Log Interpretation Techniques 81 Tpor = total porosity obtained from summing all the fluid contributions POReff = effective porosity obtained by excluding contributions prior to a predefined T2 threshold BVI = the proportion of capillary-bound water to the total volume CBW = the proportion of the clay-bound water to the total volume FFI = free fluid index. The proportion of non-clay-bound or non-capillarybound fluid to the total volume. POReff = (BVI + FFI)/100. PERM = permeability obtained by applying various standard equations The most commonly used equation for permeability (Coates equation) is as follows: PERM = ((Tpor C ) ) * ( FFI BVI )2 4 where C is a constant (typical values are around 0.10). By definition BVI + CBW + FFI = Tpor. Above the transition zone, i.e., at a Pc value of >100 psi, it should be true that: Sh ( hydrocarbon saturation ) = FFI Tpor . Below this Pc value, FFI will comprise both the free water and the free hydrocarbon. An example of an NMR log, from a hypothetical formation lying well above the FWL (free water level), is shown in Figure 5.4.1. Fluid differentiation with the tool may be performed by either the differential spectrum method (DSM) or the shifted spectrum method (SSM). DSM works by varying the wait time Tw, thereby exploiting the different T1 times for different fluid types. SSM works by varying the echo time Te, thereby exploiting differences in the diffusivity (D) between different fluids. This is particularly applicable to gas/oil differentiation, since the value of D for gas is much higher than that for oil or water. Typical acquisition parameters for the tool are as follows: Normal T2 mode: • Number of echoes: 600 • Wait time: 1.3 seconds (or longer if a significant polarization correction is required) • Echo spacing: 0.32 ms • Logging speed: 600 ft/hr (i.e., 2 measurements at each depth increment)
    • 82 CUMPDR9 30 0 CUMPDR8 30 0 CUMPDR7 30 0 CUMPDR6 30 0 CUMPDR5 30 0 CUMPDR4 30 0 CUMPDR3 30 0 CUMPDRB 30 0 CUMPDR1 30 0 GR 0 GRP1 150 Well Logging and Formation Evaluation PHJT2 30 “D” SEMP2 .2 2000 SEDP2 .2 2000 MPER .2 2000 GR5 30 SPLF2 OIL 45 PU - 15 30 RHOB2 MBV1 1.95 2.95 30 0 HYDR 0 30 0 WATER 0 30 0 MBV1 0 30 0 3950 3975 4000 Figure 5.4.1 Example of an NMR Log DSM mode: • Two wait times are chosen in order to detect a component with a long T1, such as gas or light oil. SSM mode: • Two Te times are used to detect a component with a different D value, such as gas.
    • 83 Advanced Log Interpretation Techniques Bound water mode: • Number of echoes: 100 • Wait time: 0.2 ms • Echo spacing: 0.32 ms Total porosity mode: • 50 measurements of 10 echoes are made • Echo spacing: 0.32 ms • Wait time: 5 ms Note that data can also be acquired in a stationary mode. Often a few stationary measurements, with very long wait times and numbers of echoes, are acquired as a check that the logging speed being used is not too fast. NMR properties of reservoir fluids vary with pressure, temperature, salinity, and viscosity. Table 5.4.1 gives some general values that may be of use. Through further TDAs, the software can produce the oil and gas saturation. If resistivity data are input into the software, a secondary measurement of hydrocarbon saturation is made. Limitations in the physics of early-generation tool were: 1. The difficulty of generating a uniform magnetic field over the parts of the formation from which the measurements were being made (the T1 and T2 times are dependent on the strength of the static field) 2. Problems with the static magnetic field magnets at downhole temperatures 3. Problems with logging speed when the relaxation times could be as long as many seconds Table 5.4.1 Typical NMR Properties of Reservoir Fluids Fluid Brine (100 kppm) at downhole conditions (200 bar, 125°C) Natural gas at downhole conditions Oil, viscosity 10 cp T1 (seconds) T2 (seconds) D (10-9 m2/s) 20–25 20–25 18–22 2–3 — 170–180 0.1 0.1 0.1
    • 84 Well Logging and Formation Evaluation 4. The difficulty of obtaining sufficient depth of investigation for the measurements to be useful for saturation determination. Most of the early drawbacks with the tool have been overcome. However, it still suffers from the following limitations: • • • • • • • The tool response is still severely affected by the presence of any diamagnetic or paramagnetic ions such as arise from any iron in the mud or formation, or manganese/vanadium. These will have a great effect on the relaxation times of the hydrogen nuclei. The tool is typically far more expensive to run than a conventional logging suite, which will normally be run in addition to it anyway, due to lack of confidence in the tool and the need to keep consistent with earlier logs in the field. Logging speed is typically 800 ft/hr, less than half that of conventional logging (1800 ft/hr). Hence, there is additional rig time, adding to the total cost. There are still temperature limitations and a lack of slimhole tools available. An LWD (logging while drilling) version of the tool is still in the test phase. The depth of investigation is still far shallower than the deep-reading resistivity tools. Early claims asserted that the tool offered a superior measurement of formation permeability; however, in practice the permeability was derived from an empirical equation involving the FFI and the Tpor. In most cases the “global” calibration parameters were found to be inapplicable, requiring a local calibration in each formation against core data. It has not been conclusively demonstrated that the tool offers a more cost-effective or accurate evaluation in standard simple reservoirs where conventional techniques and models are being applied. Where it is hoped that the tool may offer a direct advantage over traditional techniques lies in the following areas: • • • Identification of zones previously missed due to high percentage of clay-bound water that would nevertheless flow dry hydrocarbons A more accurate determination of porosity, particularly in complex lithologies Advanced facies discrimination in formations where conventional logs are not capable of discrimination
    • Advanced Log Interpretation Techniques • • • • 85 A better measurement of permeability than currently possible using traditional poroperm-type plots In-situ measurement of oil viscosity Differentiation of oil/gas zones The elimination of the need to run nuclear sources in the hole Overall, it may be said that some petrophysicists really believe in the future of NMR logging and see such a tool eventually replacing conventional logs. Others point to the fact that NMR logging has been around for 15 years and has offered few real advantages in most fields. I have seen many NMR logs in which the tool shows oil in known water legs and both gas and oil both above and below the GOC (gas/oil contact). I have also seen permeabilities differ by a factor of 10 or more when compared with core-calibrated values derived from poroperm relationships. However, I have also seen the tool explain why some zones, with high total water saturation, are capable of producing dry oil. Therefore, there are situations in which NMR can offer real advantages, but running the tool should be justified on a case-by-case basis, and not just from a need to be perceived as “high tech.” 5.5 FUZZY LOGIC “Fuzzy logic” is a technique that assists in facies discrimination, and that may have particular application in tying together petrophysical and seismic data. In this chapter, the basic technique will be explained, together with a worked example to illustrate the principle. Consider a situation in which one is using a GR (gamma ray) log to discriminate sand and shale. With the conventional approach, one would determine a cutoff value below which the lithology should be set to sand and above which it should be set to shale. To use fuzzy logic, one would do the following: 1. In some section of the well where sand and shale can be identified with complete confidence, one would generate a “learning set,” that is, create a new log in which the values are set to 0 or 1 depending on whether the formation is sand or shale. 2. Over the interval defined by the learning set, one would separate all the bits of GR log corresponding to sand and shale, respectively. 3. For the sand facies, a histogram would be made of all the individual GR readings. To this distribution would be fitted a mathematical function (most commonly a normal distribution) that would capture the
    • 86 Well Logging and Formation Evaluation mean and spread of the data points. This is often called a “membership function.” 4. The same would be done for all the shale values, generating a new membership function with its own mean and spread. 5. Both membership functions would be normalized so that the area underneath them is unity. The resulting distributions would look like Figure 5.5.1. Now, supposing one were trying to determine whether a new interval of formation, having a GR reading of x, belonged to the class “sand” or “shale.” Using the functions shown in Figure 5.5.1, one would simply enter the graph at the GR axis at the value x and read off the relative probabilities of the interval belonging to either class. The interval would be assigned to the one having the greatest probability. Moreover, one can assign a confidence level based on the relative probabilities. Having understood the principle of fuzzy logic with one variable, it is easy to see how it might be extended to more than two classes (e.g., sand, silt, and shale) and with more than one input variable (e.g., GR, density, neutron). Since it is not practical to plot more than two variables on a graph, the actual allocation is performed in a computer program in the N dimensional space corresponding to the N variables. Obviously for the method to work well, it is necessary that the membership function not 0.05 0.045 Relative probability 0.04 0.035 0.03 0.025 sand 0.02 shale 0.015 0.01 0.005 0 –0.005 0 50 100 150 GR (API) Figure 5.5.1 GR Distributions for Known Sands and Shales
    • Advanced Log Interpretation Techniques 87 overlap much in the N dimensional space they occupy. Also, the method does not work well with parameters that vary gradually with depth. The advantage of the method over other approaches such as neural networks is that one is able to see, through plotting the membership functions with respect to a certain variable, whether or not it is applicable to include a certain variable or not. Also, the method can generate a confidence level for the output classification, as well as a “second choice.” The use of fuzzy logic has been mainly in acoustic and elastic impedance modeling, where one can investigate whether or not, for instance, there is any acoustic impedance contrast between oil- and gas-filled sandstones. If there is, the membership functions may be used as input to a seismic cube for allocating facies types to parts of the seismic volume, thereby showing up potential hydrocarbon zones. Fuzzy logic may also be useful to allocate certain facies types to the logs, as for instance a basis of applying a different poroperm model. In my experience with using fuzzy logic, I have often found that one starts out with too many facies, which then are found to overlap each other. Also, the effect of adding more log types as variables, which may be only loosely related to the properties one is interested in, is generally detrimental. In many respects, fuzzy logic is similar to the statistical analyses packages described earlier. In common with these, it has the advantage over deterministic techniques in that it can handle a lot of variables impartially and simultaneously. However, also in common with those packages, it can easily generate rubbish unless great care is taken with the input. Exercise 5.2. Fuzzy Logic 1. Set up a fuzzy logic model to distinguish between net and non-net on the basis of GR using the data from the core as a learning set. 2. Apply the model to the lower half of the entire logged interval. Compare the average net/gross with that derived using the conventional analyses. 5.6 THIN BEDS Conventional petrophysics relies on the logs being able to resolve the individual beds in order to determine such properties as Rt, rhob, and GR. While examination of any core will reveal many features that are far below the resolution of all but imaging logs, this commonly does not pose any
    • 88 Well Logging and Formation Evaluation serious problems, provided that any variations average over an interval used for evaluation. Problems arise where the variations initiate a nonlinear response in the tool used to evaluate them. The most common example of this is the effect of thin shale beds on the resistivity log. For beds that are perpendicular to the borehole, the resistivity may be approximated by: 1 Rt = 1 Rshale + 1 Rsand Hence a small amount of conductive shale may significantly lower the Rt seen by the tool. When saturations are calculated, they may make the zone appear predominantly water bearing and not perforated, when in fact the water saturations in the individual sand beds are very low and the zone is capable of producing dry oil. Note that if the laminae are sufficiently wide, so that they are still resolved by the density log (~1-ft resolution), the problem can easily be overcome by using a J function approach, as described in Chapter 3. If the laminae are on the millimeter scale, then they will not be resolved by the density log and other approaches should be adopted. First of all it is necessary to identify the laminated zones and to determine the proportion of sand to shale. The most reliable way to identify laminated sand is through direct inspection of the core. Measurements should be made to determine the relative thicknesses of the sand and shale layers as a function of depth. If this is done, then it is recommended to assume common properties for the sand, with the porosity taken from a core average and the saturation derived using a saturation/height function from core capillary pressure measurements. If core is not available, then common ways to identify laminated sands are: • • • • Use of borehole images derived from either resistivity or ultrasonicbased tools Inspection of the microresistivity, which may show rapidly varying behavior (although this may also be due simply to variations in the borehole wall) Measurement of resistivity anisotropy, either from comparison of induction with laterolog types of devices or by running special induction tools with perpendicular coils, which may be indicative of laminated sequences The presence of strong shows while drilling in zones appearing to be nonreservoir on the logs
    • 89 Advanced Log Interpretation Techniques Note that the situation of the laminae being perpendicular to the borehole represents the worst case in terms of the suppression effect on the resistivity. Where the borehole is inclined to the bedding, the resistivity is affected less, and there are published equations for determining the resistivity measured as a function of the orientation angle and Rshale, Rsand. Another technique that has been applied is the Thomas-Stieber plot. This will now be explained. Start by considering a clean sand with porosity fi and containing water with a hydrogen index of HIw. There are three ways in which shale can be introduced (Figure 5.6.1): 1. Laminae of pure shale may be introduced in the proportion Vlam, with Vlam increasing until the point at which the formation becomes 100% shale. 2. Dispersed shale may fill the existing porespace until the point at which the pores are completely filled with shale. The total volume percentage of shale is given by Vsh. 3. Structural shale may replace the quartz grains while the primary porosity remains the same. The total volume percentage of shale is given by Vsh. The way these processes will affect fd and fn may be predicted as follows. Let the shale porosity be denoted by fsh and the clean sand porosity be denoted by fcsa. The HI of the shale is denoted by HIsh. Assume that Shale Distribution within Sandstones shale Laminated fsh fcsa shale shale shale Dispersed Structural Figure 5.6.1 Distribution of Shales Within Sandstones
    • 90 Well Logging and Formation Evaluation the shale and quartz have a similar matrix density and that the formation is water bearing. Laminae: fd = Vlam * fsh + (1 - Vlam ) * fcsa fn = Vlam * (HI sh + fsh * HI w ) + (1 - Vlam ) * fcsa * HI w where Vlam is the volume fraction of laminated shale. Dispersed: fd = fcsa - Vsh * (1 - fsh ) fn = HI sh * Vsh + HI w * fd where Vsh is the volume fraction of shale. Structural: fd = fcsa + Vsh * fsh fn = fcsa * HI w + Vsh * fsh * HI w + Vsh * HI sh This is displayed graphically on a fd / fn crossplot in Figure 5.6.2. Depending on the nature of the shale, the behavior can be seen to follow different trends. Therefore, if such a plot is made over a section of formation, and the 100% shale and sand points are identified, it may be possible to differentiate the type of shale fill that is occurring. Note that since oil and water have a similar HI, a similar behavior would be observed in an oil reservoir. A greater deviation would be observed in a gas reservoir since the HI for gas is much less than that for water. Thomas and Stieber’s method can be extended to other logs besides the density and neutron. For instance, a similar behavior would be expected if the total porosity (PHIT) from the density (fd) were plotted against compressional velocity (Vp) or GR. For a PHIT/GR approach, assuming also that the sand laminae may contain dispersed shale but that structural shale is not present, the relevant equations are: GR = (1 - Vlam ) * (GR sand + Vdis * GR shale ) + Vlam * GR shale
    • Advanced Log Interpretation Techniques 91 Figure 5.6.2 Thomas-Stieber Plot for Discriminating Dispersed/Laminated Shale PHIT = (1 - Vlam ) * (fcsa - (1 - fsh ) * Vdis ) + Vlam * fsh where Vdis is the volume fraction of dispersed shale in the sand laminae, and Vlam is the volume fraction of laminated shale. This should lead to a plot as shown in Figure 5.6.3. Above we have two equations in two unknowns (Vlam, Vdis), which can be solved provided that GRsa, GRsh, fcsa, and fsh are all known or can be picked from the crossplot. The sand porosity fsa may then be determined using: fsa = fcsa - (1 - fsh ) * Vdis . Vdis may be shown to equal [1 - [(GR sh - GR 0 (GR sh - GR sa )] (1 - Vlam )) * (GR sh - GR sa ) GR sh . In terms of known variables, fsa is given by: fsa = [ A * (1 - fsh ) - B * ( PHIT - fsh )] [(1- fsh ) - B * (fcsa - fsh )] where A = (GRsh - GR)/(GRsh - GRsa) and B = GRsh/(GRsh - Grsa).
    • 92 Well Logging and Formation Evaluation Clean sand point (GRsa, fcsa) Lamina ft Shale point (GRsh, jsh) Dispersed shale point (GRsa+Vdis*GRsh, fsa) 0 100 GR 0 Figure 5.6.3 Total Porosity vs. GR for Laminated/Dispersed Shale System In practice, it may be difficult to differentiate Vlam from Vdis with much accuracy. In some areas it is also possible to impose an additional empirical constraint relating GRsa to GRsh. Having determined fsa, a conventional Archie, Waxman-Smits, or capillary curve approach may be used to determine water saturation, Sw. Conventional formation pressure/sampling tools may be capable of identifying producible zones if one is lucky with the probe placement. Clearly it is preferred to run the tool in a packer-type mode when testing such zones. The only way to be completely sure whether a zone might be producible is through production testing. In this event I would recommend perforating the longest zone possible to give the best possible chance of encountering producible zones. In one field I have worked in, the oil contained in missed laminated sequences was such that some blocks in the field had a larger cumulative production than the calculated STOIIP. However, when the field was reevaluated, it was found that using conventional petrophysics but removing the cutoffs that had previously been applied had the effect of more than doubling the STOIIP. In many cases it may be true that the effect of including the nonreservoir shale laminae as net sand roughly compensates for the oil volume lost from overestimating Sw in the sands (caused by the effect on Rt of the shale laminae). However, when the shale laminae are small compared with the sands, the STOIIP will tend to be underestimated. Conversely, if the zone is predominantly shale but all treated as being net, the STOIIP may be overestimated.
    • Advanced Log Interpretation Techniques 93 Exercise 5.3. Thin Beds 1. Make a Thomas-Stieber plot using fd and fn. Identify the clean sand and shale points and establish the types of clay that are present (structural, dispersed, laminated). 2. Do you consider that your evaluation in this well is affected by thin beds? 3. Also make a plot using fd and Vp. Do you learn anything additional from this plot? 5.7 THERMAL DECAY NEUTRON INTERPRETATION Thermal decay time tools (TDTs) are used in cased holes in order to detect changes in the formation saturations occurring with time. Most commonly these changes arise from: • • Depletion of the reservoir and zones becoming swept with either water from the aquifer or injection water Formation of movement of the gas cap in the reservoir The tool works by injecting neutrons, generated in a downhole minitron, into the formation. These neutrons get captured by atoms in the formation, most principally chlorine, which then yield gamma ray pulses that may be detected in the tool. Through the use of multiple detectors, the tool is able to differentiate between the signal arising from the borehole and that of the formation. The components of the formation may be distinguished on the basis of their neutron capture cross-sections, measured in capture units (c.u.), denoted by S. The contractors provide charts to predict the values of S for different rock and fluid types. Typical values are: Sm = 8 c.u. (sandstone), 12 c.u. (limestone) Sshale = 25–50 c.u.; the value of S measured by the tool in a 100% shalebearing interval Soil = 8 c.u. Sgas = 2–10 c.u. Swater (fresh) = 22.2 c.u. Swater (200 kppm NaCl at 250°F) = 100 c.u.
    • 94 Well Logging and Formation Evaluation The S measured by the tool is assumed to be a linear sum of the volume fractions of the components times their respective S values. Clearly the accuracy of the tool in differentiating oil and water is dependent mainly on the contrast in S between the oil and water. Hence the tool works well in saline environments and poorly in fresh environments. Even in a saline environment it might be found that small changes in the input parameters result in a large change in Sw. Hence the tool can give very unreliable results unless some of the water saturations are already well known in the formation. The tool also has a limited depth of investigation, sufficient to penetrate one string of casing but not always two. It is essential to have a completion diagram of the well available when interpreting the tool, so that the relevant positions of tubing tail, casing shoes, and tops of liners are known. Where the tool is used in time-lapse mode in a two-fluid system, clearly the variables relating to the nonmovable fluids drop out and changes in Sw can be calculated on the basis of only (Sw - Shydrocarbon). Some of the equations that may be used for interpreting the tool will now be derived: Two-component system without time-lapse mode: S = (1 - Vsh - f) * S m + Vsh * S shale + f * Sw * S w + f * (1 - Sw ) * S h (5.7.1) where Vsh denotes the total volume fraction of shale. From which: Sw = [( S - S m ) - f * ( S h - S m ) - Vsh * ( S sh - S m ) [f * ( S w - S h )] (5.7.2) Two-component system in time-lapse mode: S 2 - S1 = ( Sw 2 - Sw1 ) * ( S w - S h ) * f (5.7.3) From which: Sw 2 = Sw1 + ( S 2 - S1 ) [( S w - S h ) * f] (5.7.4) For a situation in which gas replaces oil with constant water saturation in time-lapse mode: S 2 - S1 = Sg * ( S g - S o ) * f (5.7.5)
    • 95 Advanced Log Interpretation Techniques 0 GR 150 depth (ft) .45 1.95 X150 neutron –0.15 0 density 2.95 sigma 50 0 0 por shpor 0.25 0.25 X200 X250 Figure 5.7.1 Example of Interpreted TDT Log From which: Sg = ( S 2 - S1 ) [( S g - S o ) * f] (5.7.6) where Sg is the new gas saturation appearing. An example of an interpreted TDT is shown in Figure 5.7.1. In this example, constants were chosen as follows: Sw Ssh Ssa Sg Vsh = capture cross section of water: 100 cu = capture cross section of shale: 50 cu = capture cross section of sand: 5 cu = capture cross section of gas: 7.8 cu = shale volume fraction, derived from GR using GRsa = 15 and GRsh = 90 API f = porosity from openhole logs
    • 96 Well Logging and Formation Evaluation Exercise 5.4. Thermal Decay Neutron Example Consider a formation with the following properties: Ssh = 25 c.u. Smatrix = 8 c.u. Sw = 60 c.u. So = 8 c.u. f = 0.25 Vsh = 0.2 S measured by the tool is 15. 1. What is Sw? 2. Suppose that the value of Ssh can only be estimated to an accuracy of ±5. What is the uncertainty in Sw resulting from this? 5.8 ERROR ANALYSES In an ideal world, the net/gross, porosity, and saturation would be accurately known in all parts of the reservoir. In practice, one is trying to determine the properties based on measurements performed in a number of wells in the field, each subject to measurement error. Hence it is important to realize that there are two completely different and independent sources of error in petrophysical properties across a field. Firstly, there are errors arising from tool accuracy, sampling, and the petrophysical model, which will affect zonal averages as measured in individual wells. Secondly, there are errors arising from the fact that these properties are only “sampled” at discrete points in the field. Whether or not properties such as porosity and net/gross are mapped over the structure, or if the well data are used to make an estimate of the mean values, the result is uncertainty, which in some cases can be huge. We will first deal with errors in the zonal average properties as measured in a particular well. I believe the most rigorous way of dealing with measurement error is through the use of Monte Carlo analysis. This method has the advantage of not requiring any difficult mathematics and is easily implemented in a spreadsheet. In this example, we will attempt to estimate the error in the average properties for a simple sand that is assumed to follow an Archie model. The basic principle is that instead of choosing point values for all the input parameters, we will allow them all
    • Advanced Log Interpretation Techniques 97 to vary randomly between defined ranges, calculating the resulting values of net/gross, porosity, and saturation many times. By analyzing the resulting distributions, we can estimate an uncertainty range for each. Starting with the net/gross, we will assume that the net sand is determined from a cutoff applied to the GR. Assume that we are using a cutoff of GR = 50 API, below which the formation is designated as sand. From inspection of the GR we may conclude that in fact the GR cutoff could have been chosen as lying anywhere between 40 and 60. Next we import the GR for the interval into a spreadsheet down one column. Over successive columns we will set the cell to 1 or 0 depending on whether the log is determined to be sand or shale at the depth increment. By summing the 1’s in each column and dividing by the number of depth samples, we will determine the net/gross for a particular cutoff run. Above the second column, the value of the cutoff chosen randomly for that run will be calculated. The formula will look something like: = 50 + 10 * (0.5 - rand( )) where rand() returns a random number between 0 and 1. Hence in the cell will be calculated a cutoff value lying randomly between 40 and 60. Let this result be denoted as GRco. In the cells below we will determine whether or not the depth increment should be designated as sand or shale. We would fill in: = if ($A 2 –2 < B$2, 1, 0 ) where it is assumed that the GR is in column A and GRco is in cell B1. This formula is then copied down all the cells. Below the last depth increment (line N) we would put in the formula: = AVERAGE(B2:BN ) The whole of column B should then be copied to columns C, D, E, and so on, ideally at least 50 times. Now we should have 50 separate measurements of net/gross. In another part of the spreadsheet, take the mean and standard deviation of these net/gross values. In ExcelTM this can be done with the functions AVERAGE() and STDEVP(). The mean is likely to be close to that determined using a nonrandom cutoff. Two standard deviations can be used as an estimate of the uncertainty of the error in net/gross. Note that we have
    • 98 Well Logging and Formation Evaluation assumed that any inaccuracy in the GR reading itself is incorporated within the range we assumed for GRco. The next step is to calculate the uncertainty in porosity. Consider the equation: f = (rm - density) (rm - r f ) . We will do exactly the same as we have done with the net/gross, i.e., import the density readings into column A of a spreadsheet. At this stage it is advisable to remove all the intervals that are designated as nonreservoir. Determine allowable ranges for rm and rf and at the top of each column determine the values to be used in each run. If the allowable range for rm is, say, 2.65–2.67 g/cc, the equation would look like: = 2.66 + 0.01 * (0.5-rand( )) (denote this as RHOM, and likewise for RHOF). Down column B insert the equation: = ( RHOM - $Ax ) ( RHOM - RHOF ). Copy column B 50 times across the spreadsheet. Average each column at the bottom and then take the mean and standard deviation of the distribution of average porosities as you did for the net/gross. The uncertainty in the average porosity may be taken as two standard deviations. Since the porosity equation is linear, the mean porosity should be the same as that calculated through fixed fluid and matrix densities. Finally we will deal with saturation in an identical manner, although there are a few complexities. From Archie: Rt = Rw * f - m * Sw n . However, we wish to finally derive a porosity-weighted saturation, so it is better to use the equation: Swpor = ((rm - density) (rm - r f )) * {Rt * ((rm - density) (rm - rf )) m Rw ( -1 n ) } . (5.8.1) Note that this equation is equivalent to Swpor = f* Sw, substituting the porosity equation in Archie.
    • Advanced Log Interpretation Techniques 99 Above each column, it is necessary to derive randomly generated values for rm, rf, m, n, and Rw between allowable ranges. At this point it should be noted that applying a range to both m and Rw is not really fair if they have been determined through a Pickett plot, since any error in one will probably be corrected by adjusting the other so that the points still go through the waterline. Hence I would apply a range to one of the two parameters only if no water sand had been available for calibration and Rw has been chosen purely from produced water samples or regional correlation. At the bottom of each column, average the Swpor and then take the mean of all the runs and the standard deviation as before. The mean Sw is given by (SWPOR)average/(POR)average and the uncertainty in Sw is given by the standard deviation of SWPOR divided by (POR)average. Finally one should have arrived at mean and standard deviations for net/gross, porosity, and saturation that fully take into account uncertainties in all the input parameters. If you are using a saturation/height relationship instead of Archie, the same process can be applied, but choose allowable ranges for your a and b values instead of m, n, and Rw. I do not believe it is necessary to take into account error in the poroperm relationships, Swirr, or fluid densities, since these would be compensated for when making the log(J)-log(Swr) plot. If, however, the saturation/height relationship is derived entirely from core, you could consider adding a term to accommodate the uncertainty in s.cos(q). The second stage of the process involves looking at the uncertainties in the mean values of these parameters for individual reservoir units over the entire field. At this stage it is probably useful to digress a bit and cover some elements of basic sampling theory. Imagine that one is trying to estimate the mean value of people’s IQ by randomly sampling n people from a parent population of N individuals. Say the parent population has a mean IQ of M with a standard deviation of SD. The best way to estimate M is to take the mean of the IQs measured on the sample of n people, denoted by Mn. Statistical theory states that if the SD of the sample of n people is Sn, the SD of the mean of the parent population is Sn / ÷–. n If the accuracy of each individual IQ measurement is d, the overall uncertainty (one standard deviation) in the value of M is given by: uncertainty in M = (( Sn )2 n + d 2 ) . (5.8.2) Hence, if we are trying to determine mean and uncertainty in the mean of, say, the porosity in a reservoir unit over the entire field, this may be
    • 100 Well Logging and Formation Evaluation estimated as follows. Take the mean and standard deviation of the various average porosities as measured in all the wells. Say these are denoted by (POR) and (POR)SD. From the Monte Carlo analyses, one has denoted an uncertainty in the individual zonal average porosities of d. The best estimate of the average porosity over the entire field is POR and the uncertainty in POR is given by: uncertainty in POR = ((( PO R )SD )2 n + d2 ). (5.8.3) where n is the number of wells. For determination of STOIIP, many parameters, including net/gross, porosity, and Sw may be input as distributions to a further statistical package that will use Monte Carlo analysis to come up with a global probability function for the STOIIP (or GIIP). Most programs either require only a minimum and a maximum value for the parameters or require a mean, standard deviation, min, and max. In the former situation, it is recommended to take ± twice the uncertainty as calculated above as the min/max. All values within this range are treated as being equally likely. If a min, max, and SD are required, it is recommended to use the uncertainty calculated above as the SD and to take three SDs on either side of M as min/max, usually referring to absolute minima/maxima (zero probability of values lying outside) with something like a normal distribution about the mean. These are not the same min/max as referred to in a boxcar distribution, in which they are just ranges outside of which the values are unlikely to fall within a confidence of about 70%. A few concluding remarks about error analysis. With regard to the Monte Carlo analyses, it is always necessary to use good judgment when considering whether the uncertainties resulting are to be considered reasonable. An experienced petrophysicist should already have a good feel for the uncertainties in the average zonal parameters he is presenting, which should roughly agree with those derived from the spreadsheets. The sampling theory presented above assumed that n, the number of samples, is large. If this is not the case (e.g., a structure penetrated by two wells), then the results need to be treated with caution. Care should also be taken in the event that all the wells are crowded in one part of the field and there are large areas unpenetrated. This effectively means that the sampling is not random. Of course, wells are never drilled “randomly” on purpose, although looking at the actual locations of wells drilled in mature fields, they may approximate randomness rather well!
    • Advanced Log Interpretation Techniques 101 In many field static models currently being developed, all the net sand, porosity, and permeability are input from each well in the form of logs and geostatistics applied to determine the fieldwide averages, sometimes also using stochastic approaches (e.g., regarding the distribution of sand bodies). In this case the sampling part of the above becomes redundant. Such models will typically be upscaled for reservoir simulation. The saturations will then be initialized using a saturation/height function (supplied by the petrophysicist). Sometimes it is the case that the whole model is completely wrong; for instance, based on one sample, the reservoir is assumed to be oil filled when in fact there is a gas column occupying most of the reservoir. In this case the “uncertainties” presented are obviously meaningless. It is recommended that these eventualities be considered up front and, if necessary, completely separate scenarios built up, within which the theories presented above can still be applied. Exercise 5.5. Error Analysis 1. Copy the GR, density, and deep resistivity values from the top of the log to the OWC (oil/water contact) into a spreadsheet. For the purposes of this exercise, treat this as a single zone. 2. Use Monte Carlo analyses to determine the error in net/gross, porosity, and saturation for the averages derived in this well. 5.9 BOREHOLE CORRECTIONS I do not wish to cover this topic in any great detail. All the contractors provide borehole, shoulder-bed, and invasion correction charts for their tools, which can be applied as appropriate. I would like to make a few remarks about resistivity tools. Modeling of resistivity tools using analytical approaches or finite element modeling is extremely complicated. In fact no one has yet successfully modeled the combined effects of the borehole, invasion, and multiple dipping beds on the induction tool. Chart books treat each of these effects separately. Hence there is one chart for the borehole size/salinity, another for invasion, and another for (horizontal) shoulder-bed effects. Some software is available for handling dipping beds, but these programs usually assume no borehole or invasion. In reality, all these effects combine to produce a response that is extremely
    • 102 Well Logging and Formation Evaluation complicated and will affect different tools and different depths of investigation differently. Luckily, most of the world’s oil is stored in quite thick sands/carbonates. Particularly with OBM, invasion is not much of an issue, and objective hole sections are usually drilled with 81/2-in. hole, for which the borehole corrections are very small. For quicklook evaluations, except in rare circumstances, all the necessary operational decisions, working sums, and averages can be made without the use of borehole corrections other than those automatically applied during the logging process by the contractor. For STOIIP determination, as I have stated earlier, I believe the only sensible way to go is to derive a saturation/height function. If calibrated from logs, then I have presented a means whereby the effects of invasion and thin beds can be corrected for. Invasion as a phenomenon can actually be very useful when applied to time-lapse logging (i.e., relogging the same zone some hours or days later). For a start, the presence of invasion, as observed from a change of resistivity with time, indicates that permeability must be present. In some cases the change in properties can enable one to make conclusions regarding the nature of the formation fluid. One area in which correction for invasion may be very important concerns modeling of acoustic impedance for seismic modeling, which will be discussed in the next chapter. In this situation, it is essential to re-create the virgin-zone sonic and density log responses from the log data, which, because of the shallow reading nature of these tools, will not be correct without proper modeling.
    • C H A P T E R 6 INTEGRATION WITH SEISMIC While one part of petrophysics is concerned with the interface with geologists and reservoir engineers in order to produce reservoir models, a further interface exists with the seismologists to ensure that the well data are used to help calibrate and understand seismic properties. While the preparation of synthetic seismograms to tie log-formation tops with seismic horizons is a long-established technique, recent advances in far-offset seismic processing, combined with the ability to measure shear-sonic transit times, have opened up a lot of new possibilities for facies and fluid determination from seismic data. In this chapter some of these techniques will be discussed. 6.1 SYNTHETIC SEISMOGRAMS There are two elements to synthetic seismograms. The first is the derivation of the acoustic impedance (AI) from the log data, from which reflectivity may be derived. The second is the conversion of the depthrelated traces from a depth reference to a time reference so they can be compared with seismic sections. From the log data, the following may be derived: Vp = 1e6 (3, 281 * sonic) (6.1.1) where Vp is in m/s and sonic is measured in ms/ft. r = 1e6 * density (6.1.2) where r is in kg/m3 and density is measured in g/cc. 103
    • 104 Well Logging and Formation Evaluation The AI is given by: AI = r * Vp (6.1.3) where AI is in kg/m2/s. Hence an AI trace may be derived simply from the sonic and density logs. Prior to generating the AI, it is necessary to correct for any washouts; if necessary, editing the logs by hand. It is also necessary to correct the logs for any invasion. Fluid replacement is covered in Section 6.2. The logs should also be corrected for well deviation and datum level, such that they are true vertical and referenced to the same datum as used for the seismic survey. Having derived an AI trace in depth, it must be converted to seismic two-way time (TWT), which is the time that sound takes to reach a particular depth and to return to surface. Two sets of data are available to convert from depth to time. The first is the sonic log itself, which can be integrated to provide a total transit time. The second are data from WSTs (well shoot tests) or VSPs (vertical seismic profiles), which will give the TWT to certain depths in the well. The normal procedure is to use the integrated sonic log to provide the conversion between checkshot or VSP points, but to calibrate the integrated sonic to honor actual checkshot points where they exist. A calibrated sonic log will provide what is known as a TZ (time vs. depth) graph, on which the TWT relating to any depth can be derived. The TZ graph is used to convert the depth-based AI log to a time-based log, denoted AI(t). The next step is usually to convert the AI(t) trace to a reflectivity trace. This is simply done by differentiating the log with respect to time. The reason this works can be demonstrated as follows. Consider two adjacent samples having AI values AI1 and AI2. The reflectivity R is defined as: R = (AI 2 - AI1 ) (AI1 - AI 2 ). (6.1.4) If d(AI) = (AI2 - AI1) and the sampling increment is dt, then: R ~ [d(AI ) dt ] * [dt (2 * AI )] µ d (AI ) dt . (6.1.5) So we can derive R from the AI by simply differentiating it with respect to time. The proportionality is not important because the trace will later be normalized before comparing with the seismic log data. So now we have both AI(t) and R(t). These traces contain frequencies up to (1/dt), and the AI also contains a direct-current (DC) component.
    • Integration with Seismic 105 Amplitude The next step, before they can be compared with actual seismic sections, is to convolve the traces with a seismic wavelet that is representative of the frequency content and phase of the seismic signal. Because of the nature of the seismic source and absorbing properties of the earth, the seismic survey will possess only a certain window of frequencies, somewhere between 10 and 120 Hz. The seismic wavelet is also “minimum phase” (i.e., it has a main peak occurring sometime after the event that caused it). Seismic processing can largely convert the signal from minimum to zero phase (a process called whitening) such that the wavelet is symmetric about the event with a central peak. However, it cannot replace the frequencies lost by the seismic process. In order to make the well-derived AI(t) and R(t) comparable with the seismic log, it is necessary to convolve them with a zero-phase wavelet. Mathematically this is done by applying a zero-phase filter. One such example is a Butterworth filter (Figure 6.1.1), specified by four frequencies, such that ramps occur between the min/max frequencies and the middle frequencies, between which no attenuation is applied. After filtering, any DC components will be removed, and the traces, if plotted in a traditional “var-wiggle” format, may be compared directly with the seismic traces originating from around the wellbore. Figure 6.1.2 shows an example of logs converted to time and a synthetic AI trace. What we have now are synthetic seismograms (commonly called synthetics). The frequency content of the seismic log may be roughly known, but it is usual to experiment with different types of filters until the char- Frequency (Hz) Figure 6.1.1 Example of Butterworth Filter
    • 106 0 Well Logging and Formation Evaluation GR 150 Time 0 ms 1.95 density 2.95 200 DT 0 4 AI 18 AI FILT 50 100 150 200 250 300 350 Figure 6.1.2 Example of Synthetic Seismogram acter of the synthetic seismograms matches that of the seismic. Depending on what kind of normalization has been applied to the seismic traces, a similar normalization can be applied to the filtered AI(t) and R(t) traces to make them look similar. Note that since the AI(t) trace is now something like a sine wave, and the R(t) trace is based on the differential of AI(t) (i.e., a cosine wave), the AI(t) trace, if moved up or down by a quarter of a wavelength, will look very similar to the R(t) trace.
    • Integration with Seismic 107 Big increases in AI(t) (“hard kicks”) will result in a large peak. In the SEG (Society of Exploration Geophysicists) convention, this is displayed as a black loop (to the right). Similarly, decreases (“soft kicks”) will display as white loops (to the left). A similar convention is used for R(t). The depths corresponding to major changes in lithology, and hard/soft kicks, which are usually also formation tops, will have been converted from depth to time along with the AI log. These may then be overlain on the seismic to see if they correspond to seismic events. It is at this stage that the shape of the synthetic is important. Because of problems with “statics” (seismic shifts due to shallow events) or gas effects, events may have become shifted on the seismic so that they do not match the synthetic. By comparing the seismic and synthetic and matching up each loop, it may be possible to apply a static shift to the synthetic so that the two match up. Then the position on the seismic log of formation tops as seen in the well may directly tie to the seismic section. Whether or not this works will depend on how big the AI contrasts resulting from lithology changes are, and the quality of the log data and seismic analysis. I once spent two years exclusively tying synthetics to seismic sections from logs from all around the world. In my experience the method worked very well where there were major boundaries, goodquality seismic data (usually from offshore), and reasonable log data. A near-perfect match was obtained in about 5% of cases. In about 50% it was possible to tie at least one event with confidence. It is worth remembering that given the typical frequency content of seismic (say, 70 Hz) and the formation velocities (say, 5000 m/s), one would only expect to be able to resolve events having a minimum thickness of half a wavelength, i.e., 0.5 * 5000/70 = 36 m. Below this thickness, the logs will start interfering with each other and the situation becomes much more complicated to interpret. It should also be remembered that even if two random traces are compared with one another, they can be expected to match up about half the time. I have seen a supposedly good match proclaimed for well and seismic log data that were afterward found to have come from different countries! When modeling a particular seismic anomaly (e.g., the pinch-out of a GOC [gas/oil contact] against the top of a structure), it is sometimes necessary to create artificial logs by successively removing a particular section of log or changing the thickness of a sand body. This can be done quite simply in the time or depth domain and the resulting traces filtered as before to create an artificial seismic section in which the effect can be modeled.
    • 108 Well Logging and Formation Evaluation Table 6.1.1 T–Z relationship Depth (m) 600 620 640 660 680 700 720 TWT (ms) 1000.0 1009.2 1018.4 1027.6 1036.7 1045.9 1055.1 In order to model the effect of different fluid fill on synthetics, it is necessary to use Gassmann’s equations, which will be described in the next section. Exercise 6.1. Synthetic Seismogram 1. Use the sonic and density logs to derive AI for the test1 well. 2. Use Table 6.1.1 for T-Z conversion. 3. If you have a filtering package available, apply the following zero phase Butterworth filter: 10–20–70–90 Hz 4. On the filtered AI, do you see any effect due to the OWC? 6.2 FLUID REPLACEMENT MODELING Fluid replacement is a central part of AI modeling or creating synthetic seismograms. In essence it involves predicting how the sonic or density log will change as one porefluid replaces another. Unfortunately, the equations used to do this, developed by Gassmann, are cumbersome to apply. They also require input data that may not be readily available. Below is presented a step-by-step menu for doing a fluid replacement, which can be applied to model the change due to any combination of water, oil, or gas with another combination. For the initial case, it is assumed that the following logs are available: • • • RHOBinit = density log, in g/cc DTP = compressional velocity, in ms/ft DTS = shear velocity, in ms/ft
    • Integration with Seismic Definitions: Ko,g,w KF,init,final Kmatrix Umatrix Kgrain Soi,gi,wi Sof,gf,wf RHOFinit,final RHOoil,gas,water Por VPinit,final VSinit,final VPinit VSinit AIinit 109 = bulk modulus of oil, gas, water measured, in Pascals (Pa) = bulk modulus of combined fluid measured, in Pa = bulk modulus of matrix, in Pa = shear modulus of matrix, in Pa = bulk modulus of individual grains, in Pa = initial saturation of oil, gas, water (as fraction) = final saturation of oil, gas, water (as fraction) = initial/final combined fluid density, in g/cc = fluid density of oil, gas, water, in g/cc = porosity (as fraction) = initial/final compressional velocity, in m/s = initial/final shear velocity, in m/s = 1e6/(3.281 * DTP), in m/s = 1e6/(3.281 * DTS), in m/s = 1000 * RHOBinit * VPinit, in kg · m-2 · s-1 For the following, a gas/water system is assumed, but the method works equally well with oil/water. = 1/[Sgi /Kg + Soi /Ko + Swi /Kw] [K in Pa] = RHOgas * Sgi + RHOwater * Swi + RHOoil * Soi [RHO in g/cc] = (RHOM - RHOBinit)/(RHOM - RHOFinit) = sqrt(KFinit /RHOFinit)/30.48 = RHOBinit * ((VPinit * 30.48)Ÿ2 - 1.3333 * (VSinit * 30.48)Ÿ2)/Kgrain X2 = 1 + Por * Kgrain/KFinit - Por Kmatrix = Kgrain * (X1 * X2 - 1)/(X1 + X2 - 2) Umatrix = RHOBinit * (VSinit * 30.48)Ÿ2 [matrix shear modulus] X3 = Umatrix /(Kmatrix * 1.5) Matrix Poisson ratio (Mpoi) = (1 - X3)/(2 + X3) KFinit RHOFinit Por VFinit X1 Having determined Kmatrix and Umatrix, the new Vp and Vs can be determined as follows: RHOFfinal RHOBfinal KFfinal VFfinal Beta = RHOgas * Sgf + RHOwater * Swf + RHOoil * Sof = RHOM * (1 - Por) + Por * RHOFfinal = 1/[Sgf /Kg + Sof /Ko + Swf /Kw] = sqrt(KFfinal /RHOFfinal)/30.48 = Kmatrix /Kgrain
    • 110 Well Logging and Formation Evaluation Table 6.2.1 Typical acoustic properties of fluids and minerals Component Brine Oil Gas Quartz Calcite Clay Vp (m/s) K (Pa) Density (g/cc) 1500 1339 609 3855 5081 2953 2.6e9 1.0e9 0.04e9 36.6e9 65.0e9 20.9e9 1.05 0.6 0.116 2.65 2.71 2.58 Shear Modulus (Pa) 0 0 0 45.0e9 27.1e9 6.85e9 X4 = Kgrain * (1 - Beta) X5 = Kmatrix + (1.3333 * Umatrix) X6 = 1 - Beta - Por + (por * Kmatrix/KFfinal) VPfinal = sqrt(1/RHOBfinal * [X5 + X4/X6])/30.48 VSfinal = sqrt(Umatrix/RHOBfinal)/30.48 AIfinal may be calculated using VPfinal and RHOBfinal as before. Typical values for constants are shown in Table 6.2.1. Exercise 6.2. Fluid Replacement Modeling Using a spreadsheet, model AI in the oil leg to create the response that would be expected if the well were entirely water bearing. 6.3 ACOUSTIC/ELASTIC IMPEDANCE MODELING Gassmann’s equations need to be used to correct logs to virgin conditions when making synthetic seismograms. However, they can also be used to predict the acoustic impedance of formations if the fluid changes from one type of porefill to another. Generally speaking, there are two approaches to AI modeling. In the first approach, the AI response of the same formation, encountered with a different porefill in different wells, may be compared and also contrasted with the response of the surrounding shales. While one would expect that the water leg would have the highest AI, followed by the oil and gas legs, this is not always the case if the reservoir quality is changing between wells. Fuzzy logic techniques are usually used to fit AI distributions to the different facies types (water bearing, oil bearing, gas
    • Integration with Seismic 111 4.500E-07 4.000E-07 Frequency 3.500E-07 3.000E-07 Shale 2.500E-07 Water Oil Gas 2.000E-07 1.500E-07 1.000E-07 5.000E-08 0.000E+00 0.00E+00 5.00E+06 1.00E+07 1.50E+07 2.00E+07 Acoustic impedance Figure 6.3.1 Comparison of Acoustic Impedance Distributions for the Same Formation Penetrated in Different Wells bearing, and nonreservoir) and compare these to see the extent to which they overlap. They will be distinguishable on seismic only if the distributions do not overlap. Figure 6.3.1 shows an example of some distributions. In the example given, which is based on real data, it may be seen that there is extensive overlap between the distributions. Also, because the formation quality was poorer in the well that encountered gas, the mean AI for the gas sands was higher than that for the water/oil zones. This illustrates the fact that lithology effects are usually an order of magnitude greater than fluid effects. In the second approach, one may use the formation as seen in just one well, and use the Gassmann equations to predict the change in AI as the porefill is changed. In the example given, a well that found the sand to be oil bearing was used to model the effect of changing the porefill to gas and water. Figure 6.3.2 shows the distributions. It remains the case that the sands would be overshadowed by the underlying shale distribution, although, as expected, the gas case shows a lower AI than the oil case, which is itself lower than the water case. The fact that we got a different result depending on whether we used modeling or real well data should sound a caution to anyone using one of
    • 112 Well Logging and Formation Evaluation 7.00E-07 6.00E-07 Frequency 5.00E-07 oil 4.00E-07 gas 3.00E-07 water shale 2.00E-07 1.00E-07 0.00E+00 0.00E+00 5.00E+06 1.00E+07 1.50E+07 2.00E+07 Acoustic impedance Figure 6.3.2 Comparison of Acoustic Impedance Distributions for the Same Formation in One Well Modeled with Different Porefills these techniques to predict, say, the presence of oil-bearing sands from seismic. While the compressional velocity does depend quite significantly on the porefill, particularly on the gas saturation, the shear velocity depends hardly at all on the fluid fill. The shear behavior of the rock may be measured with seismic to some degree by using far-offset traces. Because of the nature of sound reflecting off a surface, at high incidence angles a P wave will generate S waves when it is reflected/refracted, and these can be detected. Where only the far-offset traces are used (called AVO, for amplitude versus offset), it is possible to generate a 3-D seismic cube of elastic impedance (EI) as well as AI. The EI cube, which is mainly dependent on the shear velocity, is largely independent of fluid-fill effects. In a similar way to generating AI traces using r and Vp, it is also possible to generate EI traces from the logs using r, Vp, Vs, and knowledge of the seismic incidence angle (q). There are various published equations for doing this. Once such equation is: EI = VpŸ (1 + tan 2 (q )) * (1000 * r) Ÿ (1 - 4 * (Vs Vp ) * sin 2 (q ) * 2 Vs Ÿ (1 - 8 * (Vs Vp ) * sin 2 (q ). 2 (6.3.1)
    • Integration with Seismic 113 It is possible to use fuzzy logic in the same way as with AI to also generate membership functions for various facies as a function of their EI distributions. Hence, in the example above, it might be possible to discriminate the sands from the shales (irrespective of porefill) using the EI. Having discriminated the sands, the AI might be used to discriminate the different porefills. It should, of course, be remembered that one is still limited by the quality and resolution of the seismic. Nevertheless, in some fields, AVO techniques have provided very useful information that has led to the discovery of hydrocarbons. Exercise 6.3. Acoustic Impedance Modeling 1. Compare the AI response in the oil leg with the AI response expected if the sand were water bearing and with that from the overlying shales. 2. Do you consider it likely that you could distinguish between sand and shale from the seismic? Between oil- and water-bearing sands? Explain.
    • C H A P T E R 7 ROCK MECHANICS ISSUES While rock mechanics can be a very complicated subject, there are a few basics that all petrophysicists will need in their day-to-day work, which will be covered here. In a normal reservoir, the formation rock is subject to greatest stress from the overburden. This stress arises from the weight of rock above and can be measured by integrating the density log to surface. Since density logs are not usually run to surface, a common working assumption is that the overburden stress is approximately 1 psi/ft. The vertical strain (i.e., compaction) caused by this stress is offset by the formation pressure, which helps “support” the rock. Because the structure is usually partially open-ended, the fluid will take only a proportion of the overburden stress. However, in overpressured reservoirs where the fluid is not free to escape, the formation pressure may become close to the overburden pressure. The net effective vertical stress seen by the formation is given by: s z = Poverburden - Pformation (7.1) This is actually not the true effective vertical stress, which for given conditions of Poverburden and Pformation would result in the same strain in the sample if applied with zero pore pressure. This will now be demonstrated. Let Km equal the bulk modulus of the matrix, when the pore pressure equals the vertical stress, defined by: K m = stress strain = ( Poverburden – Pformation ) (dVm V ) . (7.2) Let Kb equal the bulk modulus of the dry rock, as measured in a normal core measurement, defined by: 115
    • 116 Well Logging and Formation Evaluation Kb = stress strain = Poverburden - Pformation (dVdry V ) . dV = dVm + dVdry = Pformation . V K m + ( Poverburden - Pformation ). V Kb (7.3) The strain is given by: dV V = Pformation K m + ( Poverburden - Pformation ) Kb . (7.4) The true effective rock stress is given by Kb* strain: * = Kb ( Pformation K m + ( Poverburden - Pformation ) Kb ) = Poverburden - (1 - Kb K m ) * Pformation . (7.5) The factor (1 - Kb/Km) is usually denoted by a and called the poroelastic constant. As long as Km >> Kb, then a ~ 1 and the net effective stress is a good approximation of the true effective stress. This assumption may break down if soft shales are present as part of the matrix. Experiments on North Sea samples from the Fulmar formation have shown that values of a as low as 0.7 may be encountered. Where stress issues are likely to be important, such as where compaction and subsidence are likely to have an impact, it is recommended to make measurements of a on representative core samples. For reasons that will be explained, sz is not the pressure that should be used for SCAL (special core analysis) measurements at in-situ conditions. Because the rock is constrained laterally, there are also lateral stresses (sx and sy), which, because of the firmness of the rock, will be less than the vertical stress. In a normal reservoir, where there is no significant difference in sx or sy, they are given by: s x = s y = s z * m (1 - m ) (7.6) where m is Poisson’s ratio of the rock, related to Vp and Vs via: m = [(Vp Vs ) 2 - 1] [(Vp Vs ) - 1]. 2 (7.7) The average stress (siso) experienced by the rock is given by: s iso = (s x + s y + s z ) 3 = s* (1 + m ) [3 * (1 - m )]. z (7.8) In a laboratory SCAL experiment, any confining stress is applied evenly over the sample, hence “isostatic” (or “hydrostatic”) conditions apply.
    • Rock Mechanics Issues 117 This means that less pressure need be applied than the sz calculated in the reservoir by a factor given by equation 7.4. A typical value for m for sandstones is 0.3. This means that the stress needed in the laboratory is only 0.63*sz. Often confusion arises about the relation between the different types of compressibility measured in the lab on rock samples, so this will be explained. First of all bear in mind that when compressing a sample by applying an external stress, the pore pressure may be either kept constant or allowed to vary. The pore compressibility at constant pore pressure, denoted by Cpc (= 1/Kf), is given by: Cpc = - (1 Vpore ) * ∂ Vpore ∂ Pc (7.9) where Vpore is the pore volume and Pc is the confining stress. The bulk compressibility at constant confining pressure, denoted by Cbp, is given by: Cbp = - (1 Vbulk ) * ∂ Vbulk ∂ Pp (7.10) where Vbulk is the bulk volume and Pp is the pore pressure. The bulk compressibility at constant pore pressure, denoted by Cbc (= 1/Kdry), is given by: Cbc = - (1 Vbulk ) * ∂ Vbulk ∂ Pc . (7.11) The pore compressibility at constant confining pressure, denoted by Cpp, is given by: Cpp = - (1 Vpore ) * ∂ Vpore ∂ Pp . (7.12) If modeling compaction effects arising from depletion, one is typically concerned with Cbp, since the confining pressure (the overburden) will stay constant while the pore pressure drops. In a reservoir simulation, one is typically concerned with Cpp. For measurement of porosity, cementation exponent, and permeability “at overburden,” one will typically supply the core contractor with the pressures to use. While the most important pressure is the one corresponding to the initial uniaxial stress conditions, measurement should also be extended beyond the pressure to cover any uncertainty in Poisson’s ratio and the expected conditions at abandonment. Note that the leak-off test, commonly conducted after first drilling out a casing shoe, may also provide useful information about the weakest
    • 118 Well Logging and Formation Evaluation lateral stress. In such a test, the formation is pressured up using the mud until close to the point at which it fractures. Hence, this pressure (after correcting for the weight of the column) will be equivalent to the weakest lateral stress. When Vp and Vs are measured in a well using a dipole sonic tool, one can directly calculate Poisson’s ratio, and this is normally done as standard in the log print presented to the client. Exercise 7.1. Net Effective Stress You need to decide the appropriate hydrostatic lab pressure to use for some core plug measurements. Relevant data are as follows: Depth: 12,000 ft Overburden gradient: 1 psi/ft Formation pressure gradient: 0.435 psi/ft Poisson’s ratio of the rock: 0.35 1. At what pressure should the measurements be performed? 2. Suppose you are now told that the poroelastic constant for the samples is 0.85. What pressure should you use?
    • C H A P T E R 8 VALUE OF INFORMATION It is important for petrophysicists to have a feel for the economic impact of the work they are performing and whether or not the cost of running a certain log is really justified in view of the economic benefit it will ultimately bring. In this chapter, some considerations and tools for assessing this will be explained. In a normal oilfield economic model, money is expended on exploration until a discovery is made. Following discovery, there is a development phase involving significant capital expense (called CapEx) on wells and facilities. At a certain point, money will start coming in from production and start to pay back CapEx. There will also be ongoing operating expenses (OpEx) and tax on revenues. At the payback time, the revenues will have covered the sunk CapEx and OpEx and the project starts to move into the black. At any point in time, the field will have a future value (ignoring all the sunk costs), which will be denoted as net present value (NPV). The NPV will be calculated from the production forecasts, together with assumptions about hydrocarbon prices, taxes, and future OpEx and abandonment costs. The time element in these costs and revenues is taken into account with present value accounting, which relates all cash flows to a fixed reference point. It is important to realize how information is related to NPV. Obviously the more information you have about a field, the more wisely the CapEx may be expended (e.g., in right-sizing the facilities and drilling the most cost effective wells) and the greater the revenue. However, there will be an effect of diminishing returns. This is illustrated in Figure 8.1. Due to the cost of information, the NPV will rise with information up to a point, then start to fall. Even if money is being spent on information at a steady rate, the incremental value will become less with time during the life of 119
    • 120 Well Logging and Formation Evaluation 60 50 Cost / Value 40 30 NPV+COI 20 COI NPV 10 0 –10 0 50 100 150 200 –20 Information Figure 8.1 The Value of Information the field, since there are no decisions left to be made that can lead to greater revenue on the basis of the information. An example would be acquiring a core in a field one month before abandonment—the data cannot be used to change anything, so the money is just wasted. At this point it is important to get a feel for the relative amounts of money one is talking about, at least with respect to the impact of logging. Say that very early in the life of an assumed 50-MMbbl field, prior to designing any facilities, it is decided to include nuclear magnetic resonance (NMR) logs in all the early development wells. This comes at a cost of half a million dollars, but it is assumed that the tool always gives correct results. The logs are justified because of known concerns about the conventional evaluations, since it is considered that there is a 30% chance that the stock tank oil initially in place (STOIIP) is being seriously underestimated and could be as high as 75 MMbbl. Consider what will happen if the logs are not run. The $500,000 will not be spent. There is a 70% chance that the facilities will be designed correctly and that the field will realize an NPV of, say, $500 million. There is, however, a 30% chance that the STOIIP is in fact 75 MMbbl. If this is the case, then the facilities designed for 50 MMbbl will be suboptimal and will result in deferred production and slightly less ultimate recovery factor. The economic impact of this would be such that the NPV would be only $650 million.
    • 121 Value of Information If the facilities were to be right-sized for 75 MMbbl, the NPV would be $700 million. The estimated additional monetary value (DEMV) of running the NMR logs can be calculated by: DEMV = (0.3* 700 + 0.7 * 500 - 0.5) - (0.3* 650 + 0.7 * 500 ) = $14.5M. (8.1) Obviously in this case the decision to run the NMR logs is expected to make a profit. However, note that the decision to run the NMR logs has not gained you 25 MMbbl oil, which might have a value of $500 million. It has only allowed you to make decisions that have made you develop the field more efficiently. Most of the extra 25 MMbbl would have been produced anyway. Up until now you have also assumed that the tool always leads you to the right result. Consider instead the situation in which you run the tools but have a confidence of only R (expressed as a fraction) that they will give you the right answer. Here you have opened up the possibility of building facilities for a 75-MMbbl field that is only in fact a 50-MMbbl one. In this event, say you make only an NPV of $400 million. These data may be put in the form of a decision tree (Figure 8.2). Prob NPV Reserves: 75 MMbbl Yes Reliable result? Yes –0.5 No 0.3 700 0.7 500 0.3 650 0.7 400 0.3 650 0.7 500 (fac designed for 75) 50 MMbbl (fac designed for 50) Reserves: 75 MMbbl (fac designed for 50) Log NMR? 50 MMbbl No (fac designed for 75) Reserves: 75 MMbbl (fac designed for 50) 50 MMbbl (fac designed for 50) Figure 8.2 Decision Tree
    • 122 Well Logging and Formation Evaluation Following the same logic through as before, the EMV is: DEMV = ( R * (0.3* 700 + 0.7 * 500 ) + (1 - R) * (0.3* 650 + 0.7 * 400 ) - 0.5) - (0.3 * 650 + 0.7 * 500 ). (8.2) For the case in which R = 0.5 (50% probability of being correct): DEMV = 517 - 545 = -28 Hence, your decision to undertake NMR logging has cost the company $28 million! Whenever I have seen these kinds of decision-tree calculations performed, the possibility that the data acquired may lead one to make the wrong decision is never considered. Equation 8.2 allows you to calculate the value of R at which the data acquisition becomes worthwhile. If we plot the DEMV vs. R, we get the result shown in Figure 8.3. From the plot we can see that below a reliability of 67%, the NMR tool is not worth running. The plot can also tell us what the economic benefit would be of taking steps (e.g., further tool calibration, special studies, etc.) to improve the reliability of the tool. Such plots are a persuasive means of convincing management of the benefits of data acquisition 30 20 DEMV (MM$) 10 0 –10 –20 –30 –40 –50 –60 0 0.2 0.4 0.6 R Figure 8.3 DEMV vs. R 0.8 1
    • Value of Information 123 or research campaigns. The mathematical concepts in the probability theory concerned with value of information (VOI) are discussed in Appendix 4. Generally speaking, early in the life of a field the EMV of correct information is very high. However, the negative EMV of information that is misleading may be generally even higher. This is because the loss in NPV arising from making your facilities too small is much less than the cost of making them too large—the former will most likely only cause deferment, while the latter will see unused capacity that is just lost capital. On top of that, you had to pay for the information in the first place. Late in the life of a field, the situation is completely different. You may be largely stuck with the facilities you have, so finding out that your STOIIP is 75 MMbbl instead of 50 MMbbl will not lead to any major change in your development—you will just make more money than you originally expected to. Likewise, if you find out that your STOIIP is less, there is nothing you can do about it. In essence, the value of the information becomes much smaller, while the acquisition cost remains the same. Hence, whereas in the initial example the cost of the logs was effectively negligible but the reliability was the crucial issue, at the end of field life the cost may be a major factor but the impact of unreliable data is relatively less. The golden rules for deciding whether a course of data acquisition is justified may be summarized thus: • • • • When considering the potential economic benefits from the data, you have to consider the whole field economics, and crucially what different decisions will be made as a result of the outcome, and what their impact will be. Remember that “finding” additional hydrocarbons does not necessarily have an economic value equal to the spot price of those hydrocarbons. You must factor in the probability that the information will lead you to make the wrong decision as well as the right one. Remember that there is always the option to acquire the data but to choose not to act on it if you consider it to be unreliable or if it just confirms what you already assumed. Certain data (e.g., virgin formation pressures) can be acquired only at an early stage in the field development, so it is sometimes better to acquire them even if their impact cannot immediately be assessed.
    • 124 Well Logging and Formation Evaluation Exercise 8.1. Decision Tree Analysis Consider the following scenario: You are trying to decide whether to run a thermal decay time (TDT) tool in an old producing land well. Log costs are $20,000 (including cost of mobile wireline unit, lubricator, etc.). The well is currently producing 200 barrels of oil per day (bopd) from zone A in the well. With no intervention it is anticipated that the well will produce a further 60,000 barrels. However, zone B, which was never perforated in this well, may still be producible. There is a 50% chance that it is already flushed, but if it is not, then it might be expected to produce 600 bopd a year, or a total of 180,000 barrels. After running the TDT, you have the option to decide whether to leave things as they are or to spend $1 million doing a workover, which will entail abandoning zone A. The formation water is not very saline, and there is no base TDT log to compare with. Therefore, reliability of the results is estimated at only 70%. If you were to recomplete the well, believing zone B to be not flushed when in fact it was, the well would have to be abandoned. For the purposes of the VOI exercise, assume an oil value of $20 per barrel (after tax). 1. Is the TDT justified? If not, at what level of reliability would the tool run be justified? 2. Repeat the calculations for the situation in which there is a 30% chance that the zone is already flushed, and for a 70% chance. 3. Consider the case that there is a 50% chance that the zone is flushed, but while the chance of the tool correctly indicating a nonflushed zone to be nonflushed is 70%, the chance of the tool correctly indicating a flushed zone to be flushed is only 60%.
    • C H A P T E R 9 EQUITY DETERMINATIONS Equity determination becomes necessary when part of an accumulation extends across a boundary and becomes subject to different conditions. Such a boundary may be a result of (a) different ownership of the acreage, such as occurs in the North Sea, where different groups of companies control different blocks, or (b) international boundaries. In either of these cases, it becomes necessary to determine the relevant amounts of hydrocarbons lying on either side of the boundary. Typically, following an equity determination there will be a unitization agreement, whereby a single commercial unit is formed with the aim of optimizing the total recovery of the field. Equity denotes the share of this controlling unit held by the various parties. A higher equity will involve a greater share of the profits, but also a greater share of the costs and liabilities. Since the parties involved will want to make the most profit from the field, there will usually be an attempt by each party to maximize its own equity. The process whereby an agreement is reached on how the equity is divided is called an equity determination. Especially where international boundaries are concerned, equity determinations may take many months or years to conclude and may involve significant deferment of the hydrocarbon production. Recognition of this fact, together with a tendency for field sizes to become smaller, has led to a more pragmatic approach in recent years. However, the costs of the technical work are still sizable. When a field is first discovered and there are insufficient data available to make a proper equity determination, “deemed equity” will typically be agreed upon by the parties. This is a rough working agreement to enable appraisal/development of the field to progress, with costs reallocated and recovered as appropriate following a full equity determination at a later stage. 125
    • 126 Well Logging and Formation Evaluation 9.1 BASIS FOR EQUITY DETERMINATION Clearly there are different ways to determine equity, based on the parameters likely to define the ultimate value in the field and what can easily be measured. These are: 1. Gross bulk volume (GBV). It might be argued (by the party with the greatest GBV) that the fairest way to divide equity is on the basis of the total bulk volume of hydrocarbon-bearing rock between certain horizons. This would have the advantage of being relatively quick and simple to determine, since the parties would have to agree on only the seismic interpretation and mapping procedures. However, the other parties would rightly argue that it is not the rock that has the value, but only the hydrocarbons that are produced from it. Therefore, as long as reasonably reliable methods are available to determine the hydrocarbon production from either side of the boundary, to make a determination purely on the basis of GBV would be unfair. 2. Net pore volume (NPV). This determination might be justified on the grounds that it is also relatively simple to determine. However, if the reservoir quality on one side of the boundary is relatively poor compared with the other, the contribution to production arising from the poorer side will be relatively less. Also, if one side has a relatively low relief above the free water level (FWL), then it will contain relatively less hydrocarbon. Such a determination also makes no distinction between the relative value of oil and gas. 3. Hydrocarbon pore volume (HCPV). By introducing the saturation, some of the drawbacks of an NPV basis are removed, but this would still not take into account the fact that not all the hydrocarbon may be recoverable (e.g., if it is located in very thin or low-permeability rock), as well as the relative value of gas and oil. 4. Barrels of oil equivalent (BOE). For this basis, a factor is used to convert in-situ gas volumes to an equivalent oil volume on the basis of value. This factor will reflect the local value of gas compared with oil, bearing in mind the costs of transporting it. Hence it takes into account the fact that one side may have relatively more gas than the other side. Note that there are two sorts of gas to be included: the associated gas that is produced from the oil and any free gas existing in the reservoir. Typically the same gas factor will be used for both. 5. Reserves. It might be argued that this is the only fair way to properly allocate value. However, in practice it may be impossible to agree on
    • Equity Determinations 127 how much actual production arises from the different sides of the boundary. The relative contributions will necessarily be a function of the development strategy and positions of the wells. The technical teams working in each company will typically make their own estimate of equity on all of the above bases. Having determined which one is most favorable, they will try to propose that basis to the other parties. Needless to say, one party will likely propose a particular method only because it suits it, not out of any genuine desire to “keep things simple.” In all equity determination I have been involved in, the final basis has been on BOE. This is usually the most logical basis for determination. 9.2 PROCEDURES/TIMING FOR EQUITY DETERMINATION As stated above, at the early stages of field development, the senior managers of each company will often get together and agree on a deemed equity to allow development to commence. An initial deemed equity that is low will mean that the cash contribution required for the development will be less. Therefore, it might be considered advantageous to minimize one’s deemed equity. It may often be the case that the final equity does not differ much from the deemed equity. Some companies are thus wary of pegging their deemed equity at a low value because they believe it will tend to result in a lower ultimate equity. During the deeming phase, parties may not necessarily share the same data or make well data they have acquired on their side of the boundary available to other parties if they feel it will be detrimental to their case. The managers also have to agree on who will operate the field and the infrastructure. In some cases, sunk well costs might be shared between the parties if they can be used for development purposes. All these considerations will be incorporated within a joint operating agreement subject to an interim unitization agreement. Generally speaking, the deemed equity will remain in place during the development drilling until some time around first oil (or gas). At that point there will be a provision for one or all parties to request a full equity determination. Note that it is not essential that this ever actually occur. Each party will consider how much it is likely to gain (or lose) from such a determination, also bearing in mind the costs of it. It might be that all parties, rightly or wrongly, do not consider it to be in their interests to call a full equity determination, in which case the deemed split may remain in
    • 128 Well Logging and Formation Evaluation force. There will typically be provisions made for further equity determinations during the life of the field. However, as time progresses, the incentive for determination may be less in view of the decreasing remaining value of the field and likely increasing determination costs (resulting from additional data acquired). Many methods have been used for the actual determination. Among the most common are: 1. Technical determination. In this procedure, a series of technical meetings are called between the parties, and in theory a common technical position is agreed upon from which the equity may be determined. In practice these have typically been found not to work, since each party will tend to cling to a technical case that optimizes its own position. Such negotiations have been known to go on for years, with eventual resort to the courts, resulting in a loss of value all around. Therefore, this procedure has more or less been abandoned (and rightly so). 2. Fixed equity. Early in the field life, a company that is likely to control a very high proportion of the equity may offer the minority parties a fixed percentage, which is not subject to future equity determination. This has the advantage for the majority shareholder of not having to devote a lot of resources to negotiating over a small percentage or even share data with the minority party. It may have advantages for the minority shareholder that is a small company and does not want the expense of a full equity determination. 3. Management negotiation. This is just an extension to the deemed equity procedure. The managers might recognize that some adjustment is needed to the deemed equity after some years, but they do not wish to go to the full expense of a full technical procedure. They would normally give themselves the option to call a full equity determination at some point in the future. 4. Sealed bids. Full technical determinations can also be avoided if each party makes a sealed bid, stating its equity case, to an independent third party (such as a judge). If the sum of the equity bids is less than, say, 110%, each share can be prorated downward accordingly. If the total is greater than 110%, then a full technical procedure can be instigated. 5. Expert determination. In a full technical procedure, all of the available data are handed over to an independent expert who will make the determination. In theory this sounds like the most logical way to do an equity determination. However, a number of pitfalls can arise: • The parties have to be able to agree on a suitable expert.
    • Equity Determinations 129 • Essentially all the technical work has to be reperformed, involving considerable time and cost. There may also be difficulties in transferring all the data to the expert in a format that can be readily used. • If the expert’s results significantly erode the equity of one party from that previously assumed, the results may be challenged and not accepted by the party disadvantaged. 6. Expert guidance. Rather than have the expert perform all the technical work from scratch, another option is for all the parties to submit a technical report to the expert, stating their respective cases for their proposed equity positions. By auditing the reports and analyzing the strengths and weaknesses of each, the expert may derive a new equity split. This is certainly a quicker and cheaper alternative to expert determination. However, the problem remains that a party losing equity may challenge the expert’s results. 7. Expert pendulum arbitration. This procedure is similar to expert guidance, except that the expert is asked only to choose between the various cases submitted, not interpolate between them. The idea is that it puts pressure on the parties not to make a claim for unreasonably high equity, since it will probably result in their case not being chosen. This procedure is quicker and simpler than expert guidance but suffers from the same weaknesses. It also introduces a “lottery” element that some companies might not find acceptable. In my view a management negotiation is nearly always the preferred route for equity determination, since it saves a lot of time and money, and even a full technical determination is probably not nearly as accurate as the technical experts would have us believe. However, in most of the equity determinations I have been involved in, the goodwill between companies has been lost at an early stage as a result of middle managers trying to gain points and taking a nonpragmatic view. Therefore, there is usually no alternative to a full equity determination. I am aware of one company devoting 16 man-years of work in-house to an equity determination, with additional expert/legal costs amounting to millions of dollars. I also know of a field (straddling an international boundary) that was shut in for 7 years because a unitization agreement could not be reached. 9.3 THE ROLE OF THE PETROPHYSICIST Assuming that a full technical determination is made that is related to HCPV or BOE, the petrophysicist will play a crucial role in the equity
    • 130 Well Logging and Formation Evaluation case that can be made on behalf of his company. Clearly the petrophysicist has a duty to calculate parameters using accepted and correct methodologies. However, it is perfectly reasonable that the methodologies should be chosen and applied in such a way that they happen to provide the most favorable equity position for his company. It is not always obvious how the equity will be affected by a different choice of methods and parameters. Therefore, the first step is to set up a model that will enable the effects of changing parameters and models on the equity position to be determined quickly. This is most easily done as follows. Choose a well on either side of the boundary that may be considered reasonably representative of the existing wells. In some cases, it might be necessary to choose more than one well, particularly if both gas and oil are present. Copy all the raw log data into a spreadsheet. Set up the following parameters as global variables: • • • • • • • • • • • • • • • • • • • • Vsh cutoff Porosity cutoff Grain density Fluid density, gas leg Fluid density, oil leg Fluid density, water leg Rw m n FWL Gas/oil contact (GOC) J function, Swirr J function, a value J function, b value J function, scos(q) (oil/water) J function, scos(q) (gas/water) Oil density Gas density Water density Gas factor (for converting in-situ gas volume to oil volume) Set up the evaluation of the wells in terms of the above variables so that you are able to provide proxies for the equity variables for each well as follows:
    • Equity Determinations • • • • • • 131 GBV: from the height of the total hydrocarbon-bearing column NPV: from the sum of the net footage times the porosity HCPV1: from the sum of the net footage times the porosity times (1 - Sw), Sw derived by Archie BOE1: as per HCPV1 but multiplying any gas footage by the gas factor before summing HCPV2: as per HCPV1 except using the J function instead of Archie BOE2: as per HCPV2 but multiplying any gas footage by the gas factor before summing Let the well on your side of the boundary be denoted by A, and the one on the other side by B. Determine the following parameters: EQ(GBV ) = GBV(A ) (GBV(A ) + GBV(B)) , where GBV(A) is the GBV proxy determined from well A, etc. EQ( NPV ) = NPV(A ) ( NPV(A ) + NPV(B)) EQ(HCPV1 ) = HCPV1 (A ) (HCPV1 (A ) + HCPV1 (B)) EQ(BOE1 ) = BOE1 (A ) (BOE1 (A ) + BOE1 (B)) EQ(HCPV2 ) = HCPV2 (A ) (HCPV2 (A ) + HCPV2 (B)) EQ(BOE 2 ) = BOE 2 (A ) (BOE 2 (A ) + BOE 2 (B)) . Note that if it has been decided to use more than one well on either side of the boundary, the equations can easily be extended, using relative weighting factors as appropriate. For example, if two wells are chosen for your side (A and B) but only one is available from the other side (C), and you feel that well A is likely to be twice as important as well B, you could calculate: EQ( BOE1 ) = ( BOE1( A) + 0.5 * BOE1( B)) ( BOE1( A) + 0.5 * BOE1( B) + BOE1( C)). We are not interested in the absolute value of EQ(BOE1), only its relative size compared with other proxies for equity (e.g., EQ(BOE2)) and how it varies subject to input petrophysical parameters. Which of these is
    • 132 Well Logging and Formation Evaluation the maximum will give an indication of which basis for determination is likely to be the most favorable to your company. However, we will assume for further discussions that the BOE method is decided upon as being the most reasonable basis for equity determination. At this point it is probably best to decide whether it is going to be advantageous to push for a saturation/height function approach to saturation determination or to opt for a simple Archie approach. Arguments that can be used in favor of either approach are: Pro Archie: • It is relatively easy to agree on the Archie input parameters, particularly if SCAL (special core analysis) data are available and the water leg has been logged. • Most petrophysicists will tend to average capillary (cap)-curve data a different way, and it is hard to agree on a common saturation/height function. Pro J Function: • Saturation/height functions are the only acceptable way to do volumetrics in a field. • This is the type of function that will be used for the dynamic model, so it should also be used for the volumetrics. The next step is to determine the relative weight that each of the various input parameters has on the equity. This is done by varying the input parameters sequentially within justifiable ranges and observing the effects on EQ(BOE1) and EQ(BOE2). The result might be as shown in Figure 9.3.1. These results tell you where you can hope to make the most impact on equity. If negotiating which parameters to use with partners, you can decide up front where you should make a strong case for a particular parameter being used, possibly in exchange for being more lax on another parameter. Net/Gross If the wells on your side of the boundary tend to have formations of poorer quality than those on the opposing side, it is in your interest to push for little or no cutoffs to be applied. As stated in Chapter 2, there are many good arguments for not applying cutoffs, but even those that are applied can be calculated in a way that is favorable to your side through
    • 133 Equity Determinations 0.25 0.5 gas factor n 2.63 Parameter 1.7 rhog 2.1 2.68 0.35 vshco 0.6 0.08 porco 0.15 1.8 –6.00% –4.00% –2.00% m 2.2 0.00% 2.00% 4.00% 6.00% Equity Points Figure 9.3.1 Effect of Petrophysical Parameters on Equity the way that Vsh is calculated and the Vsh cutoff that is applied. If the wells on your side are relatively good, you could make an argument that for equity, as distinct from normal calculations of HIIP (hydrocarbons initially in place), it is only reasonable to include sands that are likely to contribute to production via the base-case development scenario, rather than to include those that could conceivably produce, given some exotic recovery mechanism yet to be devised. If using a Vsh cutoff results in lower equity than using a porosity cutoff (based on the density) or density/neutron crossover, then you can make a case for using an alternative method. If net/gross is a property that will be mapped over the reservoir, you need to further consider how this mapping will be done. Within commercial contouring packages, there are different algorithms that can be used for this that will give different equity results. You might also consider it in your interest to propose just using a constant net/gross over the entire field or using a different constant on either side of the boundary. Porosity Assuming that the density log method is being used to determine porosity, you can investigate which choice of fluid density and grain
    • 134 Well Logging and Formation Evaluation density optimizes your position, provided that the values you use are still supportable by any core or regional data. I would personally not recommend using a density/neutron crossplot approach even if it gave a better equity case. However, this method is widely used in some oil companies, and if the other company proposes it (and it favors your equity position), then you might not choose to protest too strongly against the method being adopted. As with net/gross, how the mapping of porosity is performed may also have an impact on equity and needs to be considered. Saturation As discussed above, it is recommended to agree early on whether a resistivity-derived or saturation/height approach should be used. If an Archie model is adopted, then you can try to optimize your equity through the choice of Rw, m, and n. As before, any values proposed should be supportable by either core or log data. If appropriate, a shaly sand model may also be applied. However, the more complicated the methodology becomes, the harder it will be to reach an agreement with other companies as to the parameters that should be used. If it is agreed to use a saturation/height function approach, then the parameters defining the curve that optimize equity can be proposed in a similar way as with Archie. Since the proper methodology for averaging cap curves is not always well understood by petrophysicists in oil companies, by proposing a function up front, supported by the available cap-curve data, one has a good chance of getting it accepted. Fluid Contacts Since the reservoir shape will usually not be symmetric with respect to the boundary, the position of the contacts will often have a large effect on equity. Note that if a saturation/height function is being used, the FWL should be used as the cutoff for volumetric determination. If a conventional Archie approach is being used, then the hydrocarbon/water contact (HWC) is more appropriate. For reservoirs with a gas cap, moving the GOC up or down will either favor or disfavor one company. Where contacts are clearly observable on the logs, there is not much room for debate. However, where this is not the case, then there may be a wide range of possible contacts, depending on assumptions made with regard to the formation pressures, spill point, bubble point, etc. As before,
    • Equity Determinations 135 you need to propose the model, supported by at least part of the data, that optimizes your position. If a two-well model is being used to estimate the effect on equity, care needs to be taken that this is indeed representative. For complex reservoir geometries where additional accumulations open up as the HWC is deepened, it may be necessary to go to a full mapping package to properly assess the impact on equity of moving the contact one way or another. Exercise 9.1. Optimizing Equity Consider the following scenario: Well test1 is on your side of a block boundary with well test2, which is part of the same accumulation, on the other side of the boundary and controlled by another company. You have agreed to share data with the other company and to pursue a common petrophysical model for evaluating the accumulation. The test2 well has no core data. You have agreed to give a technical presentation to the other company with your recommended model for the evaluating the field. This model, if accepted by the other company, may be used as the basis for an equity determination. Perform an evaluation of the test2 well. Fully detail a technical defensible model for the evaluation of both wells which optimizes your equity position. The log data from the test2 well is in Appendix 3. Assume that the data are true vertical relative to mean sea level.
    • C H A P T E R 1 0 PRODUCTION GEOLOGY ISSUES The purpose of this chapter is to provide readers with a convenient reference for production geology as it relates to the petrophysicist’s daily tasks, though not intended to be a comprehensive guide to the wider discipline. The interface between the petrophysicist and production geologist is crucial in ensuring that: • • • • Wells are proposed and drilled in optimum locations The correct operational decisions are made while the well is drilled The field model makes optimal use of the available well data Production from the well can be properly understood in a structural context For this interface to work well, it is essential that both discplines have a working understanding of each other and a common terminology. The primary duties of the production geologist are as follows: 1. Correlate all the available well data within the production area, providing a logical and consistent formation designation scheme. 2. Prepare, in conjunction with the seismologist, geological subsurface interpretations comprising subsurface maps of key horizons and cross sections. 3. Update subsurface interpretations as new well/seismic data become available. 4. Advise on selection of new well locations. 5. Determine the gross bulk volume (GBV) of the reservoir, which may be used, in conjunction with data provided by the petrophysicist, to determine net pore volume (NPV), hydrocarbons initially in place 137
    • 138 Well Logging and Formation Evaluation (HIIP), etc. Such models may be either deterministic (i.e., using fixed distributions of reservoir properties) or probabilistic (i.e., using probability distributions for reservoir properties). 6. Create an upscaled subsurface model in digital format that may be exported to the reservoir simulator and used for dynamic simulation and reserve estimation. 7. Provide the geological background for any proposed well stimulation or secondary recovery projects. Poor communication between the petrophysicist and the geologist can sometimes lead to some expensive mistakes, particularly with respect to proposing future wells and assessment of reserves. Here are some of the major pitfalls I have come across: • • • • Where the reservoir zonation is quite coarse (i.e., one zone covers a large depth interval), there may be considerable variation of the reservoir properties over the zone. One petrophysical average may be quite inappropriate. For instance, say a zone consisted of 3 m of good-quality sand of 1 darcy permeability overlying a 100-m interval having 10-md sand. The zone, seen as an average, might appear unproducible when actually the good sand, taken in isolation, might be commercial. It could also happen that the whole zone is interpreted as commercial when in practice only a small part will ever contribute to production. Where the reservoir zonation is too fine, there is a danger of incorrect correlation between wells. This often leads to an incorrect assessment of fluid contacts or to some serious errors in estimation of reserves. Great care needs to be taken in the allocation of permeabilities in the production geologist’s model. Bear in mind that this may be done in a number of ways. The petrophysicist may apply a poroperm equation to his porosity log and supply the production geologist with a curve to use within his software. In this case the petrophysicist needs to ensure that the permeabilities in nonreservoir units are set to an agreed-upon value. In particular, very high permeabilities arising from spurious porosity values need to be edited out. The petrophysicist may supply the production geologist with a poroperm equation to apply himself. In this situation there is an even greater danger of incorrect values entering the model. If the geologist is resampling the data to a coarser depth interval before applying the equation, the resulting permeabilities are very likely to be incorrect due to the nonlinearity of the poroperm equation.
    • Production Geology Issues • • • • • • 139 The petrophysicist may supply the geologist with a constant average permeability to use for the horizon in question, or supply averages for each well. There is a danger here that either the average does not take into account known geological variation over the structure or that contouring of well averages leads to an incorrect interpretation of the areal variation. The above arguments with respect to permeabilities may also apply to water saturations. The petrophysicist may supply the production geologist with saturations in the form of either logs, averages, or a saturation/height function. It is essential that a clear audit trail for the saturations in the model be supplied. Probably the greatest source of error in the petrophysics/production geology interface lies in the realm of net-to-gross values, and this has led to some huge mistakes in the past. Probably the safest approach, where the petrophysicist is supplying the geologist with evaluated logs for inclusion within a static model, is for porosity to be set to zero in all nonreservoir units, and net/gross to be set to unity throughout. However, even this can cause problems where upscaling is occurring. It is really essential for the petrophysicist to sit with the geologist and see just how net/gross is being handled within the software used to generate the static model and how this is passed on to the dynamic model. Picking of coring points or well TD (total depth) is often done by the production geologist on the basis of his correlation. Failure to incorporate all of the petrophysical information available may often result in bad decision making. Sometimes the petrophysical interpretation itself depends on the geological interpretation. For instance, if gas/oil differentiation is not possible from the logs alone, use might be made of the known production history from neighboring wells, which will depend on the correlation. If this correlation is wrong, the fluid allocation will be wrong. It may also happen that the petrophysicist makes an interpretation of the fluids that leads to an unresolved inconsistency between wells. In this case the production geologist may be forced to introduce a fault in the structure, which may or may not exist in reality. The logging program may incorporate items that have a positive value of information (VOI) only in the event of the well being a success. Therefore, the petrophysicist, using good communication with the production geologist, may be able to save money on the well through provision of early information on the well’s results. Poor communication
    • 140 • • Well Logging and Formation Evaluation will often lead to logs being run that have no value. Conversely, where the results are unexpected, additional logging having a high VOI may also be proposed. The petrophysicist and production geologist will often be using well deviation data from different sources. It is essential that these be checked for consistency before any work on true vertical log data is shared. Both the petrophysicist and the production geologist may have access to reports and logs that are outside the domain of information shared digitally between departments. Where there is poor communication and lack of a proper library structure, it may often occur that neither has access to the most complete information that can be used to improve his models. On numerous occasions I have seen this with respect to core data. 10.1 UNDERSTANDING GEOLOGICAL MAPS 10.1.1 Basic Concepts Consider a three-dimensional surface, such as the top of a particular horizon in the subsurface. If you were standing on such a surface, there would be a direction in which the surface slopes most rapidly. Relative to north, this direction would have an azimuth, referred to as the azimuth of the dip direction. The angle between this direction and the horizontal is referred to as the dip magnitude. If we were to take a horizontal line perpendicular to this direction (called the strike line) and measure the angle going clockwise from north to this line, we would have the strike direction. These items are illustrated in Figure 10.1.1. The strike lines of the surface, when combined for a specific horizontal elevation, form contours. Maps of a surface are created by showing contour lines for fixed vertical spacing. For a smooth surface, these lines will be continuous. However, where the surface is not smooth (for instance, where faulting occurs), the lines will be discontinuous. The average dip magnitude may be measured from a contour map by taking the distance between contour lines and using the formula: tan(a ) = ( vertical contour spacing) ( horizontal contour spacing) (10.1.1) where a is the dip magnitude. Example: If the contours on a 1 : 50,000 map are every 200 m and are spaced by 6 cm, what is the dip magnitude?
    • 141 Production Geology Issues Vertical North Strike C B A East A = Dip Magnitude B = Dip Azimuth C = Strike Azimuth Direction of max dip Figure 10.1.1 Dip and Strike tan(a ) = (200 ) (50000 * 0.06) = 0.067 a = 3.8 degrees. Exercise 10.1. Dip Magnitude If the contours on a 1 : 25,000 map are every 100 m and are spaced by 4 cm, what is the dip magnitude? 10.1.2 Types of Maps The map described in the first paragraph of the preceding section concerns the depth of the top of a particular horizon and is therefore familiar to anyone used to reading geographic maps. However, mapping does not have to be limited to just the parameter of depth. The technique can be used to represent any parameter that varies areally over a structure. Other parameters that may be mapped and contoured include: • Thickness of a particular horizon: This may be either the isochore thickness (i.e., in the vertical direction) or isopach thickness (in the direction normal to the bedding plane).
    • 142 • • • Well Logging and Formation Evaluation Petrophysical properties (porosity, net/gross, saturation, permeability) or combinations of such (equivalent hydrocarbon column [EHC], NPV) Paleography, environments of deposition, facies variations Fluid properties (depth of contacts, fluid density, salinity) 10.1.3 Methods of Contouring Based on well data alone, the structure of a particular horizon is known only in discrete locations. What happens in between must be estimated. Various mathematical algorithms are available to estimate the most appropriate values of a parameter to use between locations where the values are known with certainty. These are: 1. Triangulation. Straight lines connecting data well locations are created, and along these lines the data are interpolated linearly. The subsequent contours may be smoothed using a “spline fit,” which attempts to reduce the second derivative of the curve. 2. Inverse distance. In this technique, the inverse of the distance from each known data point is used to establish a weighting to use for taking an average of the known data values. Hence, if there are n known values (Z1 to Zn), the value (Z) of the parameter at some intermediate location is determined by: n Z = Âi =1 ( Z i di ) Â n i =1 (1 di ) (10.1.2) 3. Polynomial fit. Rather than taking just the inverse of the distance, a polynomial function may be used. The coefficients of this polynomial function may be determined from the data itself by finding the set of coefficients that fits the data best for the known well locations. 4. Kriging. Kriging is an advanced technique that involves using all the data available to determine the best combination of the available data points at a particular intermediate location. In order to do this, it is first necessary to mathematically describe how the parameter in question varies between the known data points. This is done by constructing a semivariogram of the data. A semivariogram may be constructed using the formula:
    • Production Geology Issues i= N (h ) gamma(h) = (1 2 * N (h)) * Âi =1 where Vxi¢,yi¢ Vxi,yi h N(h) (Vxi¢,yi¢ - Vxi,yi )Ÿ 2 143 (10.1.3) = (known) value of the parameter at point xi¢, yi¢ = (known) value of the parameter at point xi, yi = distance from xi, yi to xi¢, yi¢ = the number of pairs that are a distance h apart. A plot of gamma(h) vs. h constitutes the semivariogram. The semivariogram captures the contribution to the total uncertainty when using a data point that is a certain distance away from the point at which the estimation is being made. Kriging involves finding a set of weighting factors (done automatically within a computer) that minimizes the total uncertainty in the estimate made at the intermediate point. Moreover, kriging also provides the variance of the estimation error from which an error map can be drawn. 10.1.4 Quantitative Analysis from Maps While maps are invaluable in helping the petrophysicist and geologist understand the areal variation of properties, they may also be used for quantitative analyses. In most petroleum engineering departments, quantitative work is always done on the computer these days. However, it is always recommended to make a reality check using more basic techniques, since lack of full understanding of how software works, or bugs in the program, can lead to erroneous results. Initially we will show how GBV may be determined from maps, then extend the concept to show how HIIP can also be determined. Consider an oil reservoir where both the top and base of the structure have been mapped, and the oil/water contact (OWC) is known. The first step is to make an area-depth map for the top of the structure. This is done by measuring the area contained within each contour, starting at the shallowest and working gradually deeper, until one is at the first contour that falls below the OWC. The area contained within this contour, since it is an irregular shape, is most often measured using a device called a planimeter, which (once the area has been completely traced) will determine the area contained therein. Obviously, since the planimeter measures only the area on the paper, a conversion has to be made using the map scale. Hence, for example, if the map is 1 : 25,000, a traced area of X square centimeters
    • 144 Well Logging and Formation Evaluation would be equivalent to a real area of (25000)2*X/(100)2 square meters. Once this has been done, a plot is made on graph paper of the area (in square metres) against the depth (Figure 10.1.2). The same is now done for the base of the structure, and the two measurements are combined. A line is drawn indicating the position of the OWC (Figure 10.1.3). By planimetering (or just counting squares on the graph paper), one can determine the area (in cm2) on the area-depth graph representing the 2500 m 2800 m 2600 m 2700 m Depth (m) 2700 m 2600 m 2500 m 2800 m Area (m2) Figure 10.1.2 Maps and Area-Depth Graphs 2500 m 1 cm = 105 m2 2600 m 1 cm = 50 m Depth (m) 2700 m Position of OWC 2800 m Area (m2) Figure 10.1.3 Gross Bulk Volume (GBV) from Area-Depth Graph
    • Production Geology Issues 145 volume of the GBV of the oil column. To convert this area into a volume: Draw a square of side 1 cm on the graph paper. Along one horizontal side, indicate how many square meters are represented by 1 cm. Likewise, on the vertical side indicate how many meters in depth are represented by 1 cm. Taking the product of these two conversion factors will yield the conversion factor to convert square centimeters into cubic meters of rock. In the example shown in Figure 10.1.3, if the area measured from the graph were 10 cm2, the GBV of the oil column would be given by: GBV = 10 *10 5 * 50 = 5 *10 7 m 3 . The technique illustrates well why there are problems with using net/gross when reservoir quality varies in depth in a reservoir. This can be seen as follows. Imagine that the horizon shown in the example consists of a very good quality sand overlying a nonproducible shale. Say the porosity in the sand is 30% and Sw = 0.10. However, taking the whole interval into consideration, the net/gross is only 30%. One would estimate the STOIIP (stock tank oil initially in place) to be: STOIIP = GBV * (0.3) * (0.9) * (0.30 ) Bo where Bo is the oil formation volume factor. Now consider what would have happened if the base of the good sand were mapped instead of the entire package. A new GBV (GBV¢) would be determined by planimetering the new area-depth graph. The net/gross corresponding to GBV¢ is now 1.0 instead of 0.30, but of course GBV¢ is less than GBV. Another estimate of the STOIIP is then given by: STOIIP ¢ = GBV ¢ * (0.3) * (0.9) * (1.0 ) Bo . Because of the nature of the way that the OWC cuts across the structure, it is certainly not the case that STOIIP = STOIIP¢. In essence, because the poor-quality rock falls preferentially below the OWC, one is underestimating the STOIIP by using the coarser interval and applying a net/gross. Conversely, if the good-quality rock had been located at the base of the sequence instead of at the top, the STOIIP would have been overestimated through using net/gross. The concept of area-depth mapping may also be applied to other parameters. For instance, if a map is made of the EHC (net*porosity*hydro-
    • 146 Well Logging and Formation Evaluation EHC (m) 0m 15 m 5m 10 m 10 m 15 m 5m Area under graph represents HIIP in m3 0m Area (m2) Figure 10.1.4 Hydrogen Initially in Place (HIIP) from Area-Depth Graph Table 10.1.1 Example of well data 1 2 3 4 5 6 7 8 9 Easting, km Northing, km Depth of Top Sand, m tvdss* 100 100.5 101 100.6 100 100 99.3 98.5 99.5 Well 100 100.7 100 99.3 99.2 101.5 99.3 100 100.6 3000 3100 3200 3100 3080 3120 3110 3220 3090 *m tvdss = meters of true vertical depth subsea carbon saturation), this may also be planimetered to yield the STOIIP directly, as is illustrated in Figure 10.1.4. Exercise 10.2. Area-Depth Graph An oil accumulation is discovered by wells penetrating the structure at the locations/depths shown in Table 10.1.1. The thickness of the sand is uniformly 325 m, with porosity 20% and average Sw of 20%. The OWC was found at 3,070 m at TVDss
    • Production Geology Issues 147 (total vertical depth subsea). Bo is 1.3 rb/stb (reservoir barrels/stock tank barrels) 1. 2. 3. 4. Make a map of the top structure and draw contours every 25 m. Construct area-depth maps for the top and base of the structure. Estimate the GBV, NPV, and STOIIP. Consider what the effect on STOIIP would be if in fact Sw were 10% for depths shallower than 25 m above the contact, and 30% for 0–25 m above the contact. 10.2 BASIC GEOLOGICAL CONCEPTS 10.2.1 Clastic Reservoirs Clastic rocks are defined as being composed of consolidated sediments formed by the accumulation of fragments derived from preexisting rocks and transported as separate particles to their places of deposition by purely mechanical agents. These fragments may be transported by water, wind, ice, or gravity. The manner of their movement may be by suspension, saltation, rolling, or solution. The effect of the transportation is to change any of the following characteristics of the fragments (or grains): • • • • • • Size Shape Roundness Surface texture Orientation Mineralogical composition Measurement of these changes may provide information on the transportation mechanism. The types of environmental deposition are as follows: Desert Waddis Aeolian dune systems Sabkhas Fluvial Alluvial fans Floodplains
    • 148 Well Logging and Formation Evaluation Channel systems Deltaic Distributary channels Delta plain Delta front Glacial Loess Tillites Varves Deep marine Turbidite fans Slumps, slides, debris flows Turbidity currents Shoreline Beaches, bars, barrier islands Coastal aeolian plains Cheniers Swamps, marshes, estuaries Knowledge of the environment of deposition is crucial to understanding how the good-quality sands, if any, are likely to be distributed over a prospect. Mineral composition of clastic reservoirs, in order of abundance are: quartz, clay minerals, rock fragments, feldspars, chert, mica, and carbonate fragment. A “clean” sandstone will comprise mainly quartz grains. This may have a porosity as high as about 40% and permeability up to 5 darcies. The presence of clays and minerals forming cement between the grains will have the effect of reducing the porosity and permeability, as will different grain distributions. The distribution of clay minerals is also related to the environment of deposition and is of particular significance to the petrophysicist, in view of the effect that clay has on permeability, conductivity, and water saturation. Clays found in sandstones are classed as either allogenic or authigenic. Allogenic clays are those that were present prior to deposition. These may take the following forms: • • • Individual clay particles, dispersed as matrix or as laminae Pellets formed from clay flocculation or excreted by organisms Clay aggregates derived from preexisting shales outside the depositional basin
    • Production Geology Issues 149 Authigenic clays are those that have formed at the same time or after deposition. Three processes are possible that lead to their presence: • • • Allogenic clays may be transformed into new types through the effect of temperature, pressure, and pH. Clay mineral may be formed by the diagenesis of nonclay minerals such as feldspars, pyroxenes, amphiboles, and micas. Clay mineral may be precipitated from porefluids. Principal clay minerals are as follows: • • • • Kaolinite, Al2Si2O5(OH)4. The clay is usually in the form of hexagonal crystals, which may be stacked to form accordion-type shapes. These structures may fill the pores and have an impact on permeability. However, the cation exchange capacity (CEC) of kaolinite is low, meaning that it absorbs relatively little water. Chlorite, (Mg, Fe)6 AlSi3O10(OH)8. Chlorite may form in many different shapes, such as plates, rosettes, honeycombs, or round growths. Typically it will coat the sand grains and pore throats, having a negative effect on permeability. However, unlike some other clays, it does react to acid, so permeability around the wellbore may be increased via acid stimulation. CEC is also relatively low. Illite, (H3O, K) (Al4Fe4Mg4Mg6)(Si7Al)O22 (OH)4. Illite can be particularly damaging to permeability when it takes the form of hairlike structures that may block the pore throats. It does react partially to strong acid. The CEC is higher than that of chlorite or kaolinite but lower than that of montmorillonite. Montmorillonite, (Na, K, Mg, Ca) Al2Si4O10(OH)2H2O. Montmorillonite has the highest CEC and has therefore the greatest effect on saturation through clay-bound water. Swelling of the clay can also cause problems when drilling with freshwater mud. The clay takes the form of crinkly coatings on detrital grains, or a cellular structure similar to honeycomb chlorite. 10.2.2 Carbonate Reservoirs Carbonates originate from the calcareous skeletons of organisms, forming bioclastic sediments. These fragments are cemented by carbonate precipitating from water. Most of the organisms lived on the bottom in shallow marine water, where algae were present. However, after dying, the organisms may
    • 150 Well Logging and Formation Evaluation have fallen to a greater depth and accumulated. Below a certain depth (4,000–6,000 m), all carbonate is dissolved as a result of the high pressure. The main difference between carbonate and clastic reservoirs is that clastic deposition requires the transportation of grains to the sedimentary basin, whereas carbonates originate within the basin of deposition. Since the effect of clastic deposition is to typically cloud the water, making the environment unsuitable for organisms relying on photosynthesis, it is usually not possible to have carbonate and clastic reservoirs coexisting. However, it is of course possible for one to be overlying another due to changes in the environment of deposition over geological time. Carbonate reservoirs comprise the following types: • • • Shallow marine carbonates. The rate of skeletal production in shallow marine water is generally high. These skeletons break down, due to action by crustaceans and fish or by turbulence. The effect is to generate carbonate sediment that may be transported to the final place of deposition. This sediment may be modified by burrowing organisms. Fecal pellets so generated may form grains, and hence result in porosity. Deepwater carbonates. Deepwater carbonates are deposited at a depth below that at which photosynthesis occurs. Typically the sediments are formed from oozes consisting of skeletons of pelagic organisms. Reefs. Reefs are built by calcium carbonate–secreting organisms growing on the remains of previous generations. The large skeletal organisms (e.g., corals) generally remain in place after death, and this may result in the formation of cavities partially filled with sediment. Most reef sediment is produced by segmented (e.g., crinoids, algae) or nonsegmented organisms (bivalves, brachiopods, foraminifera) that grow in the spaces left by larger skeletal organisms. Initially, the porosity in calcisands (i.e., matrices comprising carbonate grains) is very high (45% porosity and 30 D permeability). However, postdepositional diagenetic processes have the effect of drastically reducing this. Factors that reduce the porosity are: • • • • Cementation: precipitation of CaCO3 from the pore waters into the porespace Internal sedimentation: or filling of the porespace by sediment Compaction: grain repacking Pressure solution: dissolution of CaCO3 in one part and precipitation in the porespace of another part
    • Production Geology Issues 151 Processes that may increase the porosity are: • • Leaching: This can be fabric related or nonfabric related. Fabric-related leaching is selective due to mineralogical differences in the sediment and results in mouldic porosity. The nonfabric-related type tends to lead to large voids or karsts. Dolomitization: This is the replacement of CaCO3 by CaMg(CO3)2. Where this creates porosity, it will generally be vuggy or intercrystalline in nature. 10.2.3 Faulting and Deformation In any field, the forces acting on the sediments will be both gravitational and either extensional or compressional in any horizontal direction. Extensional features may result from (a) lack of lateral support nearby sediments, (b) movement of basement rocks, or (c) instabilities in the overburden arising from differential compaction or salt diapirs. Compressional features may result from (a) gravity-sliding of rocks over inclined basement surfaces, or (b) movement of basement blocks. The main features observed in extensional tectonics are as follows: • • • • Growth faults. These are faults in which the thickness of rocks in the downthrown block are greater than those (in the same time units) in the upthrown block. The downthrown block has subsided quicker than the upthrown block and acquired more sediment. The fault plane is usually curved upward. This is illustrated in Figure 10.2.1. Rollover anticlines. These are growth anticlines in which rock units thicken from the crest toward the flanks. The flanks subsided faster than the crest and accumulated a greater thickness of sediment. Normal faults. Movement in the basement either during or after deposition may cause faulting. In normal faulting the sediments on the upper side of the fault plane move vertically downward relative to those on the lower side of the fault plane. This is illustrated in Figure 10.2.1. Salt domes. During Permian and Triassic times, large volumes of seawater became isolated, and subsequent evaporation created thick salt deposits. Since the salt is essentially ductile where it is confined, there is a tendency for it to bulge at the weakest point through the overlying rock, forming a diapir. This diapir will cause radial fractures in the overlying rock around it. The diapir may itself form part of the seal, allowing hydrocarbons in sands to be trapped.
    • 152 Well Logging and Formation Evaluation Growth fault Normal fault Reverse fault Wrench fault Figure 10.2.1 Types of Fault Features associated with compressional tectonics are: • • • Reverse (or thrust) faults. Where the main stress in the reservoir is horizontal on one axis, with the second main stress also horizontal but normal to this (i.e., the stress in the vertical direction is the weakest), reverse faulting may occur. The frictional forces associated with rocks passing over one another are very high, and overpressures may often be associated with reverse faulting, since they provide a mechanism whereby the friction may be reduced (see Figure 10.2.1). Wrench faults. These will occur where the second highest stress is in the vertical direction and the weakest in the horizontal direction normal to the axis of greatest stress. This results in a horizontal displacement of one block relative to the other. Folding. Compressional stresses will generally give rise to faulting in brittle rock. In more ductile rock, folding is possible, and this can also
    • 153 Production Geology Issues Sand lens Unconformity trap Fault trap Pinch-out Grading trap Salt dome traps Anticline Buried reef Figure 10.2.2 Types of Trap provide a sealing mechanism. Terminology useful to folding is as follows: ᭿ Anticline: convex up ᭿ Syncline: concave up ᭿ Hinge line: the line of maxiumum curvature ᭿ Limb: area of least curvature or flank ᭿ Crest: topographically highest point ᭿ Trough: topographically lowest point ᭿ Axial plane: the plane defined by hinge lines of all horizons ᭿ Crestal plane: the plane defined by crests on all horizons The basic types of hydrocarbon trap are illustrated in Figure 10.2.2. 10.2.4 Abnormal Pressures Normal pressures are defined by taking a hydrostatic gradient (typically 0.45 psi/ft or 1.04 g/cc) from surface to a certain depth. Significant deviation from the pressure so determined would be termed abnormal. The principal mechanisms giving rise to abnormal pressures are: • • Low water table or high elevation. In mountainous areas, the water table may lie some depth below surface; and in the upper part, any hole drilled will be through dry rock. Hydrocarbons. The presence of a hydrocarbon column above an aquifer will cause the pressure at a depth in the hydrocarbon column to be
    • 154 • • • • • Well Logging and Formation Evaluation higher than if only water had been present. This is discussed in more detail in Chapter 2. Depletion. Production from a reservoir, where there is insufficient water drive or injection to replace the drained volumes, will result in a loss of reservoir pressure. To some extent, this drop in pressure will be offset by the effects of gas coming out of solution or compaction. However, these processes will always lag behind production. In some reservoirs, loss of pressure due to depletion may be very significant (up to 5000 psi) and result in severe problems during drilling of subsequent development wells. Compaction disequilibrium. During burial under equilibrium conditions, the water in the porespace is free to leave, thus ensuring that as the overburden increases, it is mainly the rock matrix that takes the weight of the overburden. If the water is not, or only partially, free to leave, some of the weight of the overburden is taken up by the porefluid, resulting in a much higher porefluid pressure than would otherwise be the case. Although shales are usually impermeable over production life cycles, the long time periods associated with deposition and burial usually cause them to expel water quickly enough for equilibrium to be maintained. Conditions in which such overpressures are likely to occur are when (a) the permeability of the shale decreases with compaction, (b) the thickness of the shale is very great, (c) the shale is structurally weak, or (d) the rate of burial is very fast. Aquathermal pressures. Where part of a system becomes isolated so that it retains a constant volume under burial, a change in temperature may result in a rapid increase of pressure. Phase changes. The volume of water in a system may increase, thereby resulting in an increase of pore pressure under conditions of (a) dehydration and dewatering of clays, in particular where montmorillonite is transformed into illite, and (b) conversion of gypsum to anhydrite. Osmosis. Where two reservoirs have different salinity and are separated by a semipermeable membrane (e.g., a clay), water will flow from the less saline to the more saline, resulting in an increase in pressure. In theory, such pressure differentials could reach 3000 psi, although this has not been observed in practice.
    • C H A P T E R 1 1 RESERVOIR ENGINEERING ISSUES 11.1 BEHAVIOR OF GASES Consider a gas for which the behavior is as shown in Figure 11.1.1. The line dividing the regions where the substance is liquid or gas is called the vapor pressure line. Hence, crossing the line from left to right, the liquid boils and becomes a gas; and in crossing from right to left, the gas condenses to form a liquid. Above the critical point (defined by Pc and Tc), there exists only a fluid phase with gas indistinguishable from liquid. An equation of state relates the pressure, volume, and temperature of a substance. For gases at low pressures and medium to high temperatures, the ideal gas law may be applied: pV = nRT (11.1.1) where p = pressure in N/m2 V = volume in m3 n = number of moles R = gas constant (8.3143 joules/Kelvin/mole or 10.732 psia.cu ft/lb mole deg R) T = temperature in Kelvin (°C + 273). This equation is useful for quickly estimating how the volume of a gas might change when taken from conditions of one pressure/temperature to another. For example, say there was an influx of 1 m3 of gas into a borehole, where the pressure was 3000 psi (4.35 * 106 N/m2) and 150°C (423 K). The volume at 2500 psi/130°C could be estimated as follows: 155
    • 156 Well Logging and Formation Evaluation Pressure Pc Critical point Liquid Gas Tc Temperature Figure 11.1.1 P-T Diagram nR = constant = P1V1 T1 = P2 V2 T2 . (11.1.2) Hence, V2 = V1* (P1*T2 / (P2*T1). In the example, V1 = 3000, T1 = 423, P2 = 2500, T2 = 403 K. Hence, V2 = (3000 * 403 / (2500 * 423) = 1.14 m3. A more correct equation for real gases as encountered in reservoirs is that of van der Waals: ( p + a V 2 ) * (V - b) = RT . (11.1.3) This equation may be expressed in the form: pV = nZRT (11.1.4) where Z varies with pressure and temperature. In order to calculate Z, it is first necessary to know, for the gas concerned, the values of the critical pressure and temperature defined by Pc and Tc. These are used to determine the reduced pressure and temperature given by: Pr = P Pc (11.1.5) Tr = P Pc . The most commonly used experimentally determined correlation between Z and Pr /Tr is that of Standing and Katz, shown in Figure 11.12.
    • Reservoir Engineering Issues 157 Figure 11.1.2 Standing and Katz Correlation The critical pressure and temperature may be measured experimentally. Some values of common gases are as follows: Pc and Tc may also be estimated from the formulae: Pc = 4.55 * 10 6 + 0.345 * 10 6 * g - 0.383 * 10 6 g 2 (11.1.6)
    • 158 Well Logging and Formation Evaluation Table 11.1.1 Properties of gasses Formula Gas Methane Ethane Propane Nitrogen Molecular Mass (kg/kmol) Pc (N/m2) Tc (K) CH4 C2H6 C3H8 N2 16 30 44 28 4.61 * 106 4.88 * 106 4.25 * 106 3.4 * 106 191 306 370 126 Tc = 100 + 167 * g (11.1.7) where g is the specific gravity of the gas, defined as: g = (density of gas at standard conditions) (density of air at standard conditions). (11.1.8) Exercise 11.1. Density of Air Calculate the density of air at standard conditions: temperature = 15.5°C, pressure = 1 atm. The molecular mass of air should be taken as 29 kg/kmol. The gas expansion factor (E) relates the volume of a fixed number of moles of gas at standard conditions relative to reservoir conditions. It is defined as: E = volume at standard conditions (15.5∞C, 1.10325 *10 5 N m 2 ) (11.1.9) volume at reservoir conditions. Using equation 11.1.4, it can be shown that: E = 1.965 * 10 -4 P ( Z * T ) (11.1.10) where P is in N/m2 and T is in K. Very often you will see this equation in oilfield units, in which case it becomes: E = 35.37 * P ( Z * T ) (11.1.11) where P is in bar and T is in Rankine (°R = °F + 460 = °K * 1.8).
    • 159 Reservoir Engineering Issues 11.2 BEHAVIOR OF OIL/WET GAS RESERVOIRS Oil reservoirs are generally multicomponent systems. In other words, there is gas present, either in solution or as free gas in the reservoir. Hence, a single-phase diagram such as that shown in Figure 11.1.1 is not appropriate. In systems that consist of more than one component, the points of phase coexistence constitute a two-dimensional region bounded by the dewpoint line and bubble point line. This is illustrated in Figure 11.2.1. Above the bubble point (in terms of pressure at a certain temperature), only liquid may exist, i.e., any gas is completely dissolved in the oil. Above the dewpoint (in terms of temperature at a certain pressure, all the hydrocarbons are gaseous. Reservoirs will typically fall into one of three categories, as shown in Figure 11.2.2. In wet gas reservoirs, the initial reservoir pressure and temperature lie to the right of the dewpoint line. Within the reservoir during production, as the pressure drops, the dewpoint is not crossed and the hydrocarbons remain gaseous. However, at surface, where the temperature is allowed to drop to ambient conditions, the dewpoint will be crossed and condensate liquid will drop out of the gas. In retrograde condensate reservoirs, the initial pressure and temperature are above the dewpoint, but during production the dewpoint is crossed in the reservoir. This leads to liquid forming in the reservoir (retrograde Pressure Liquid Bubble point line Gas/liquid ratio Critical point Dew point line Gas Temperature Figure 11.2.1 P-T Diagram for a Two-Phase System
    • 160 Well Logging and Formation Evaluation Pressure Wet gas reservoir Condensate reservoir Oil reservoir Temperature Figure 11.2.2 Types of Reservoirs condensation). Later on, the pressure may fall so that the dewpoint is crossed again and the condensate may evaporate in the reservoir. At surface a combination of liquid and gas will be produced. In oil reservoirs the initial pressure is such that all the gas is dissolved in the liquid at reservoir conditions. During depletion, the reservoir pressure may fall below the bubble point and gas may start to come out of solution in the reservoir. Even if the reservoir pressure is kept above the bubble point, when the fluids reach surface, the gas will have come out of solution to be produced as associated gas. In many structures, the pressure near the OWC (oil/water contact) is above the bubble point, but at a certain shallower depth the pressure falls below the bubble point. At this point gas will have come out of solution and formed a free gas cap occupying the crest of the structure. In this case, at least under virgin equilibrium conditions, the pressure at the GOC (gas/oil contact) is equivalent to that of the bubble point. In order to predict the behavior of the fluids in a reservoir, the properties should be measured on samples taken from the reservoir. This is called PVT (pressure, volume, temperature) sampling. Such samples will ideally be taken downhole and kept at reservoir pressure for transporting to the laboratory. Alternatively, samples of oil and gas may be taken at surface and recombined in the laboratory (“recombination samples”) to re-create downhole conditions. When sampling is performed downhole, it is essential that the fluids produced be free from contamination by drilling fluids
    • Reservoir Engineering Issues 161 and that the drawdown is such that samples are taken above the bubble point. A fundamental problem with PVT sampling and measurement is the fact that the end point of a process whereby the gas is liberated from oil is dependent on whether the gas is kept in close proximity with the oil during the liberation or is removed. In a reservoir, the gas typically remains in contact with the oil and will follow the pattern of behavior as indicated by the pressure-temperature (P-T) diagram. However, during production in a well, the gas will become isolated from the oil. This isolation changes the phase behavior of the oil left behind, modifying the P-T diagram. Hence, unless any experiment models the true processes undergone by the gas and oil during migration to the wellbore, production, and gas/oil separation at surface, it cannot accurately predict the actual production of gas and oil from a field. The parameters that are measured during PVT analysis consist of the following: • • • The oil formation volume factor (Bo). Bo is defined as the volume of reservoir liquid required to produce one volume unit of stock tank oil (i.e., oil at standard surface conditions). Depending on how “gassy” the oil is (i.e., how high the gas/oil ratio [GOR] is), Bo typically varies between 1.0 and 1.5. The gas formation volume factor (Bg). Bg is the reservoir volume of one volume unit of gas at standard conditions. Rather confusingly, while in SI units Bg is normally given in reservoir m3/standard m3, in field units it is usually quoted as reservoir barrels/standard cubic feet. Typical range (for SI units) is 0.004–0.06 r.m3/st.m3. The solution gas/oil ratio (Rs). Rs is defined as the volume units of gas that evolve from Bo reservoir volume units of oil when the oil is transported to surface conditions. It has the units of standard m3 gas/standard m3 oil, or standard cubic feet/standard stock tank barrel (stb). Where there is no free gas being produced from the reservoir, this is the same as the GOR. However, GOR is a term that is used to describe well production behavior, unlike Rs, which is essentially a laboratory measurement. If the reservoir pressure falls below the bubble point, free gas will be produced in the reservoir. Since the gas is more mobile than the oil, this will lead to a dramatic rise in the gas produced at surface, although in
    • 162 Well Logging and Formation Evaluation time the GOR will drop back once most of the free gas has been produced. Some of the free gas may move to the crest of the structure and form a secondary gas cap. Where the producing GOR in wells has become high, the oil rate can be improved by injecting water into the reservoir. This will have the effect of raising reservoir pressure, causing some free gas to redissolve, and also help displace oil to the wells. However, where water breakthrough occurs in the producing wells, the overall effect on production may be negative. 11.3 MATERIAL BALANCE Material balance treats a reservoir like a tank, with areal and depth pressure equilibrium. For undersaturated oil reservoirs (i.e., those for which the pressure remains above the bubble point), oil is produced by the expansion of the liquid phases and connate water, the shrinkage of the pore volume, and the influx of water from the aquifer. When the reservoir pressure drops from its initial value Pi to P, the formation volume oil factor increases from Boi to Bo. If the produced oil volume at standard conditions is Np (with initial volume N), the volume removed from the reservoir at downhole conditions must be Np * Bo. The expansion of the oil in place must be N * Boi N * Bo. The initial pore volume equals N * Boi /(1 - Sw), where Sw is the water saturation. If the compressibility of the water is Cw, and that of the pores is Cf, the composite compressibility C is given by: C = (Cw * Swc + Cf) (1 - Sw ) . (11.3.1) From material balance it must be true that produced volume (at reservoir conditions) = expansion of oil in reservoir + expansion of pore volume + water influx (W). Hence: N p * Bo = N * ( Boi - Bo ) + N * Boi * C * ( Pi - P) + W . (11.3.2) Porespace compressibility can be measured in the lab as part of a special core analysis (SCAL) program. Typical values range from about 15 * 10-5 l/bar for a low-porosity rock to 5 * 10-5 l/bar for a highporosity rock. Water compressibility varies with pressure, temperature, salinity, and amount of dissolved gas. An approximate value to use is 4.35 * 10-5 l/bar.
    • Reservoir Engineering Issues 163 Exercise 11.2. Material Balance of Undersaturated Oil Reservoir Consider a reservoir with negligible water drive (W = 0). Bubble point is 200 bar. If N = 107 standard m3, Boi = 1.3, Bo at 250 bar = 1.25, Sw = 0.2, Cf = 10.0 * 10-5 l/bar, Cw = 4.4 * 10-5 l/bar, and Pi = 300 bar, how much oil will have been produced when the reservoir pressure has dropped to 250 bar? 11.4 DARCY’S LAW For linear flow, an empirical equation developed by Darcy is as follows: u = - (k m ) * (∂ p ∂x + r * g * ∂z ∂x ) (11.4.1) where: u = flow velocity in m/s k = permeability in m2 (note that 1 darcy is 10-12 m2) m = viscosity, in Pa (note that 1 cp = 10-3 Pa) p = pressure of flowing phase, in Pa z = vertical distance upward, in m x = distance in the direction of flow, in m g = gravitational constant (9.81), in m/s2 r = density of the flowing phase. For purely horizontal flow over a distance L (having pressure drop DP) with area A, this equation reduces to: Q = - (k m ) * DP * A L (11.4.2) where Q is flow rate measured in m3/s. The above equation also approximately applies in so-called Darcy units, where Q is in cc/sec, k is in darcies, A is in cm2, m is in cp, P is in atmospheres, and L is in cm. The
    • 164 Well Logging and Formation Evaluation approximation originates from the fact that 1 atm is in fact 1.01325 * 105 Pa, and not 105 Pa. In oilfield units, and incorporating the conversion from downhole to surface conditions via Bo, the equation becomes: Q = -1.127 * 10 -3 * (k m ) * DP * ( Bo * L ) where Q is in stb/day, k is in md, A is in ft2, m is in cp, P is in psi, and L is in ft. For radial flow into a borehole, incorporating the porosity and effect of fluid/pore compressibility, Darcy’s equation becomes: (1 r ) * ∂ ∂ r (r*∂ p ∂ r ) = ∂ p ∂ t * f * C * k m (11.4.3) where r = radial distance from the center of the borehole (in m), f = porosity, and C = composite compressibility, in 1/Pa. The composite compressibility is given by: C = Co * (1 - Sw ) + Cw * Sw + Cf (11.4.4) where compressibility is expressed as Co for oil, Cw for water, and Cf for porespace. A system is said to be in a steady state when pressure does not vary as a function of time, i.e., ∂p/∂t is zero. A system is said to be in a semisteady state when ∂p/∂t is a constant. Every closed finite system that is produced at a constant rate will asymptotically approach a semi-steady state. An infinitely extended system will never approach a semi-steady state. Equation 11.4.3 has solutions depending on the boundary conditions that are applied. For a situation in which the boundary of the area being drained is maintained at a constant pressure, a steady state will eventually develop for which: P - Pw = (Q * m (2 * p * k * h)) * (ln(re rw ) - 1 2 ) where: P = mean pressure in the drainage area, in Pa Pw = bottomhole flowing pressure, in Pa Q = flow rate in reservoir, m3/sec h = thickness of reservoir, in m (11.4.5)
    • Reservoir Engineering Issues re rw k m 165 = drainage radius, in m = wellbore radius = permeability, in m2 = viscosity, in Pa. For a situation in which the boundary is sealed and the pressure is dropping linearly with time, a semi-steady state exists that has the solution: P - Pw = (Q * m (2 * p * k * h)) * (ln(re rw ) - 3 4). (11.4.6) Note that P and Pw both vary with time, but their difference remains constant. Since the wellbore radius is always very small compared with the drainage radius, it is possible mathematically to study the theoretical case in which the wellbore radius becomes infinitessimally small, whose solution is: Pi - Pf = Q * m ( 4 * p * k * h) * Ei ( x ) (11.4.7) where Pf is the pressure of the flowing phase, Ei(x) denotes the exponential integral, and x = k * r2 4 * t k = f*m *C k. The exponential integral, Ei(x), is given by: Ei ( x ) = - g - ln( x ) - S( - x )n n!*n where g is Euler’s constant (1.781). If t > 25 * f * m * C * r 2/k, the summation term becomes less than 0.01 and can be ignored; then we can give the approximate version of equation 11.4.7 as: Pi - Pf = - (Q * m ( 4 * p * k * h ) * ln( g * k * r 2 4 * t ). (11.4.8) Solving for t/r2: t r 2 = (1 4) * g * k * exp( 4 * p * k * h * D P (Q * m ) where DP = Pi - Pf. (11.4.9)
    • 166 Well Logging and Formation Evaluation Exercise 11.3. Radial Flow Calculate the time taken for the flowing pressure to fall by 30 bar in the vicinity of a well subject to the following flowing conditions: k = 300 md, h = 30 m, Rw = 15 cm, f = 0.20, m = 10 cp, C = 2*10-4/bar, Q = 200 res.m3/day, 11.5 WELL TESTING The equations given in the previous section can be used to predict flow performance in ideal cases. However, in reality, flow near the wellbore is influenced by formation damage caused by drilling and production processes (precipitation, fines moving, gas blocking, water dropout, etc.). The pressure difference between the bottomhole well flowing pressure and the reservoir pressure, DP, relates to formation properties via a formula of the form: DP = Q * m * J (2 * p * k * h) (11.5.1) where J may be considered a dimensionless productivity. In the case of steady state flow, J = ln(re /rw) - 1/2; and for semi-steady state flow, it is ln(re /rw) - 3/4. The productivity index (PI) is defined as: PI = Q DP . (11.5.2) The effect of formation damage is that the PI is smaller than one would expect from theoretical values, or DP is greater than one would expect for a given Q. This damage may be quantified by introducing a term called S such that: DP = Q * m * ( J + S ) (2 * p * k * h) . (11.5.3) If the radius of the damaged zone is r1, and it has permeability k1, it may be shown theoretically that S is given by: S = (k k1 - 1) * ln(r1 rw ). (11.5.4)
    • Reservoir Engineering Issues 167 This is subject to the following assumptions: • • • • Flow is radially symmetric. Porosity and compressibility are not affected by the damage. Pressure is continuous over the boundary between the damaged and virgin zones. The damaged zone is in the steady state and the region outside is in either the steady or semi-steady state. The effect of stimulation through acid, steam, reperforation, etc., will be to reduce S. Of particular interest to the petrophysicist is how the product k * h is derived from a well test, since this will have to be reconciled with values derived from logs. During the initial phase of testing a well, the semi-steady state has not been reached, and a transient state will prevail. In this regime the following equations apply: 2 * p * k * h * DP (Q * m ) = (1 2 ) * ln( 4 * td g ) + S (11.5.5) where td is the dimensionless time given by: 2 td = k * t (f * m * C * Rw ) . (11.5.6) If two measurements are made of P and t, kh can be derived from: kh = (1 2 ) * Q * m * ln (t1 t2 ) (2 * p * ( P2 - P ) . 1 (11.5.7) Having determined kh, S may be determined from equation 11.25. Note that in the equations presented, SI units must be used throughout, and Q is in reservoir volume, not standard volume. In reservoir limit testing, a well is produced sufficiently long for the semi-steady state to be reached. Once semi-steady state is reached, the well pressure will decline linearly with time, and the rate of change of pressure with time will be constant. In the semi-steady state: Q = - C * A * h * f * (dP dt ). (11.5.8) Hence if measurements of P vs. t are made, the drainage area A can be deduced. Most commonly, wells are tested by letting them flow for a
    • 168 Well Logging and Formation Evaluation certain period of time, then shutting them in and observing the buildup of pressure with time. If the well is flowed for a period T, then shut in, the pressure behavior will simply follow that of the pressure observed in a well flowed from t = T subtracted from that of a well flowed from t = 0. Note that the pressure observed thereby becomes independent of S, which cancels out. When analyzing buildups, it is common, in addition to defining the dimensionless time, to introduce the dimensionless pressure Pd given by: Pd = 2 * p * k * h * ( Pi - P) (Q * m ) . (11.5.9) Then it can be shown that: 2 * p * k * h * ( Pi - Pw ) (Q * m ) = Pd (Td + Dtd ) - Pd ( Dt d ). (11.5.10) In this equation, Pd(Td + Dtd) represents the pressure at a dimensionless time Td + Dtd, converted into a dimensionless pressure using equation 11.5.9. For an infinite reservoir: Pd = (1 2 ) * ln( 4 * td g ). (11.5.11) Hence 2 * p * k * h * ( Pi - Pw ) (Q * m ) = (1 2 ) * ln[(Td + Dtd ) Dt d ]. (11.5.12) This is the basis of the so-called Horner plot. If pressure measurements are made at two different times (P1 and P2), it can be shown that: k * h = (1 2 ) * Q * m * ( L1 - L2 ) [2 * p * ( P2 - P )] 1 (11.5.13) where: L1 = ln (T + Dt1) / Dt1) L2 = ln (T + Dt2) / Dt2). The extrapolation of the pressure on a Horner plot yields the initial reservoir pressure (Figure 11.5.1).
    • 169 Reservoir Engineering Issues Q T + dT T Time dT Pi Pw Ln[(T + dt)/dt] Figure 11.5.1 Pressure Buildup Analysis Table 11.5.2 Example of pressure buildup Time (hr) 0 0.5 1.0 1.5 2 2.5 3 4 6 8 10 12 Pressure (bar) 205.32 217.89 221.95 223.05 223.53 223.81 223.96 224.20 224.48 224.68 224.82 224.96
    • 170 Well Logging and Formation Evaluation Exercise 11.4. Horner Analysis A well is tested under the following conditions: Flowing period = 24 hours Production rate = 200 stm3/day Bo = 1.2 h = 10 m rw = 0.1 m f = 0.25 m = 1 cp C = 0.0003 1/bar. After the flowing periods, the well is shut in and the following pressures recorded downhole as shown in Table 11.5.2. Using the data, determine the initial reservoir pressure and the permeability.
    • C H A P T E R 1 2 HOMING-IN TECHNIQUES Homing-in techniques are used when the position of one borehole relative to another needs to be known. Reasons why relative positions may be important are: • • • In relief well drilling for blowouts, it may be necessary to intersect one borehole with another to enable the blowout well to be killed. Where survey data are unreliable, there may be a need to avoid collision between boreholes or to pinpoint the location of one well in relation to another. Some production/injection schemes require wells to be a fixed distance apart. The principal methodology available for homing in comprises electromagnetic and magnetostatic techniques. While there has been research in the past on acoustic homing-in techniques, these have not been found to be successful. 12.1 MAGNETOSTATIC HOMING IN Magnetostatic homing-in techniques use the fact that steel placed in a borehole usually has some remnant magnetization that causes a disturbance to the local magnetic field of the Earth. By running a sensitive magnetometer in open hole in a well close to another that has steel in it, this magnetic disturbance may be detected. Interpretation of the magnetic field detected as a function of depth can, in some cases, yield an accurate estimate of the distance and direction of the target well. Basic limitations on the usefulness of such methods are the generally short range over which 171
    • 172 Well Logging and Formation Evaluation such disturbances may be detected (typically <15 m) and the uncertain nature of the magnetization. Since standard surveying tools, such as MWD (measurement while drilling), now possess accurate magnetometers, there is no need for dedicated tools for magnetostatic homing in. However, interpretation from such tools is not straightforward. It is hoped that the following will provide some useful techniques, as well as a better understanding of magnetic surveying in general. 12.1.1 Magnetization of Steel Casing/Drillstrings As a result of the manufacturing process, or from subsequent magnetic inspection or mechanical shocks, the steel used in making casing, drillpipe, or accessories (e.g., jars, drill collars, bits) usually acquires a degree of magnetization. The simplest model for describing the field due to such a magnetization is in terms of a superposition of magnetic point monopoles of either north or south polarity. A north monopole is defined as one for which the lines of magnetic flux flow symmetrically and spherically toward the pole; for a south pole, the lines of flux flow similarly away from the pole. Although free monopoles are never observed in nature, in the vicinity of a pole, of which the opposite pole is far removed, the field will be dominated by that of the near pole, and the far pole may be ignored. When two or more poles are a similar distance from the point of measurement, the field is a linear superposition of the monopole field due to each of the individual poles. The magnetic field due to a monopole in a Cartesian reference system is as follows: Fx = mr * M * x ( 4 * p * r 3 ) (12.1.1) Fy = mr * M * y ( 4 * p * r 3 ) (12.1.2) Fz = mr * M * z ( 4 * p * r 3 ) (12.1.3) where: r = ( x2 + y2 + z2 ) , in m M = the pole strength, in Webers (Wb) Fx, Fy, Fz are the field strengths, in Tesla (T) mr is the relative magnetic permeability of the medium x, y, z are the distances, in m, from the monopole to the measuring point along the x, y, and z axes.
    • 173 Homing-in Techniques It is usually assumed that mr is 1.0, so this term is commonly disregarded. The system used to define the x, y, and z axes is such that x is grid north, y is grid east, and z points vertically downward. 12.1.2 Interpretation of Magnetic Anomalies When a three-axis magnetometer is run to measure the magnetic field, the field components Bx, By, and Bz in the Cartesian system are projected onto the three axes of the tool. In the tool’s frame of reference, the z axis lies along the borehole and the x and y axes, as a result of tool rotation, are arbitrarily oriented in the plane (called the sensor plane) perpendicular to the borehole. If accelerometers are also present in the tool, the angle between the x axis of the tool (called the toolface) and the highside (HS) of the hole is also known, thus making it possible to convert the x and y components in the tool’s reference system to that of a system whereby x lies in the HS direction, and the y magnetometer is 90 degrees clockwise from this in the sensor plane (called the highside right, or HSR, direction). Once this rotational transformation has been made, the readings of the magnetometer are said to be in the highside reference system. Bx and By are replaced by Bhs and Bhsr, respectively. Bz remains unchanged, although to avoid confusion will be termed Bax. If the inclination and azimuth of the hole are known, a further transformation can be made to convert Bhs, Bhsr, and Bax to Bx, By, and Bz in the Cartesian reference system. These quantities are illustrated in Figure 12.1.1. The field measured by a magnetometer in the survey well is the vector sum of the fields due to any anomalies and to the Earth. In interpreting Survey well path HS Fhs Fxy Fhsr F Sensor plane Fax HSR AX Figure 12.1.1 Fields in the Highside Reference System
    • 174 Well Logging and Formation Evaluation any magnetic anomalies, the Earth’s field must first be subtracted from the magnetometer readings. The basic procedure for locating the position of a target well, using a three-axis magnetometer combined with accelerometers, is therefore as follows: 1. Using the accelerometer data, the raw magnetometer readings are converted to the highside reference system. 2. The Earth’s magnetic field is derived in the highside reference system (Ehs, Ehsr, Eax) as a function of depth in the survey well. Note that while the Earth’s field may be constant, the components in the survey well will vary if the well’s inclination and azimuth vary. 3. The Earth’s components are subtracted from the magnetometer components to derive the components of the disturbance (denoted as F). Hence: Fhs = Bhs - Ehs (12.1.4) Fhsr = Bhsr - Ehsr (12.1.5) Fax = Bax - Eax . (12.1.6) Having calculated F, it is convenient to derive the following quantities for comparison with theoretical models: Fxy = 2 2 ( Fhs + Fhsr ) (12.1.7) Ftot = 2 2 2 ( Fhs + Fhsr + Fax ) (12.1.8) where DFax = the rate of change of Fax with respect to alonghole depth HSdir = the angle between F, as projected into the sensor plane, called Fxy, and the HS (clockwise from HS to Fxy) AXdir = the angle between F and the hole axis (from hole axis to F). The above five quantities may be plotted as a function of alonghole depth in the relief well. 4. The disturbance to the Earth’s field, defined by the components of F, must be interpreted in terms of a superposition of monopoles along the target well. This is done by forward modeling as follows. From an assumed set of pole strengths and positions in terms of alonghole
    • 175 Homing-in Techniques depths in the target well, the magnetic field, in the highside reference system of the survey well magnetometer, is modeled. This involves using equations 12.1.1 to 12.1.3, together with survey data from both wells. In order to match the measured and modeled sets of data as functions of depth, it is necessary to move the assumed position of the target well with respect to the relief well and to vary the assumed distribution of poles on the target well until a best fit has been found. Weaknesses in the above approach are as follows: 1. Effect of smearing. The point monopole model may not apply if the magnetization has become smeared along the axis of the pipe. The effect of this is that estimates of the distance made will be too large. 2. Poorly defined HS. If the relief well is nearly vertical, the HS direction ceases to be defined, and it is not possible to make the correct subtraction of the Earth’s magnetic field in the sensor plane. This problem can be avoided by ensuring that the survey well is drilled with a few degrees deviation. 3. Nonlinear well paths. It is highly recommended that the survey well be drilled on a constant deviation and azimuth when passing close to the target. This is because normal surveying of the well path is not possible while the magnetometers are affected by the disturbance. While this problem could be solved through running a gyro in the survey well, this is not normally practical in open hole. Also, visualization of the survey and target wells is much harder if the highside reference system does not remain fairly constant. 12.1.3 The Earth’s Field It will now be shown how the components of the Earth’s magnetic field in the highside reference system may be derived. Let the Earth’s magnetic field be defined by: • • • Eh = horizontal component of Earth’s field Ev = vertical component of Earth’s field Declination = angle clockwise from true north to magnetic north. The Cartesian components of the Earth’s field vector are given by: Ex = Eh * cos(declination ) (12.1.9)
    • 176 Well Logging and Formation Evaluation Ey = Eh * sin(declination ) (12.1.10) Ez = Ev . (12.1.11) From the survey well deviation data it is possible, for each survey point, to define a Cartesian unit vector RVEC, along the well trajectory. The HS direction is derived by first considering a unit vector 90 degrees clockwise from HS in the sensor plane, called HSR. Ê 0ˆ HSR = Á 0˜ Ÿ RVEC Á ˜ Ë1 ¯ (12.1.12) where Ÿ denotes the vector product (see Appendix 4). The HS vector becomes: HS = HSR Ÿ RVEC (12.1.13) The three vectors HS, HSR, and RVEC must be converted to unit vectors (i.e., divided by their magnitude) to define the highside reference system. These are then denoted by HSŸ, HSRŸ, and RVECŸ. The Earth’s components in the highside reference system are: E ax = ( E.RVEC Ÿ ) * RVEC Ÿ (12.1.14) where . denotes the scalar product. E hs = ( E.HSŸ ) * HSŸ (12.1.15) E hsr = ( E.HSR Ÿ ) * HSR Ÿ . (12.1.16) 12.1.4 Converting Survey Data to the Highside Reference System The raw magnetometer tool components Bx, By, and Bz are converted to the highside reference system as follows. The survey tool’s accelerometer readings will be Ax, Ay, Az. The orientation of the toolface with respect to the HS direction is given by: q = arctan( Ay Ax ). (12.1.17)
    • 177 Homing-in Techniques Table 12.1.1 Determining HSTF from accelerometer data Ax Ay HSTF + + - + + - 180 - q 180 - q -q -q The arctan function will normally return a value of q between -90 and +90 degrees, and we are interested in the clockwise angle (between 0 and 360 degrees) between the HS and the toolface, denoted by HSTF. To derive HSTF from q, the transformation must be applied as shown in Table 12.1.1. Bhs and Bhsr are given by the following equations: Bhs = Bx * cos(HSTF ) - By * sin(HSTF ) (12.1.18) Bhsr = Bx * sin(HSTF ) + By * cos(HSTF ). (12.1.19) The field due to a monopole, as measured in the highside reference system of a survey well, may likewise be modeled by replacing the components Ex, Ey, and Ez in the above equations with Fx, Fy, and Fz, as given by equations 12.1.1 to 12.1.3. In terms of the measurements made of Ftot, Fxy, Fz, DFz, and AXdir, the behavior will be as shown in Figures 12.1.2 to 12.1.4. Similarly for a dipole, consisting of a north and a south pole of equal strength, the behavior will be as shown in Figures 12.1.5 to 12.1.7. Note that in this example the axis of the dipole is parallel with that of the survey hole. If the survey hole passes closer to one pole than to the other, or if the poles are of different strengths, the behavior of the files will not be symmetric. In general, where more than one pole is present, it is necessary to model the field for different configurations and try to match with the measured data. This may be done by an automated procedure with a computer program. 12.1.5 Quicklook Interpretation Methods Where the field is dominated by one pole, quicklook methods may be applied to estimate the shortest distance and direction to the pole.
    • 178 Well Logging and Formation Evaluation 0.002 0.0015 Field (T) 0.001 Ftot Fxy Fax 0.0005 0 960 980 1000 1020 1040 1060 1080 1100 1120 1140 –0.0005 –0.001 depth Figure 12.1.2 Field Due to a Monopole: Ftot, Fxy, Fax Consider a general case in which a survey well passes a target well with the closest approach distance of d (see Figure 12.1.8). Let a monopole be present on the target at a height of t above the point of closest approach. The point at which the sensor is closest to the pole AXdir will become 90 degrees. Note that this is not necessarily the point at which the wells themselves are closest together. Also, although one well passes the other, HSdir remains constant with alonghole depth in the survey well. The shortest distance between the survey well and pole (x) may be determined from measuring the width of the Ftot curve at half the maximum intensity and dividing by 2. Hence: x = DFtot 2. (12.1.20) Alternatively, the Fxy curve may be used, but the formula becomes:
    • 179 Homing-in Techniques 0.00025 0.0002 Field gradient (T/m) 0.00015 0.0001 DFax 0.00005 0 950 1000 1050 1100 1150 –0.00005 –0.0001 Depth Figure 12.1.3 Field Due to a Monopole: DFax x = DFxy * (0.652 ). (12.1.21) Similarly, the separation of the two stationary points (zero gradient) on the Fax curve is related to x by: x = DFax 2. (12.1.22) This may also be measured by finding the separation of the two zero crossing points of the DFax curve. If the separation of the two points on the AXdir curve on either side of the point representing AXdir = 90 degrees at which AXdir = 135 degrees and 45 degrees is measured, x may be found from: x = DAX dir 2. (12.1.23)
    • 180 Well Logging and Formation Evaluation 200 180 160 AXDIR (deg) 140 120 100 AXDIR 80 60 40 20 0 960 980 1000 1020 1040 1060 1080 1100 1120 1140 Depth Figure 12.1.4 Field Due to a Monopole: AXdir In order to find the closest approach of the two wells, do as follows. Consider that the survey well has an inclination a. If the vector pointing toward the pole (r) is an angle q from the HS direction in the sensor plane, the angle between r and the horizontal is given by: f = arcsin([1 - cos(q )] * sin(a ). (12.1.24) The TVD (true vertical depth) of the pole is given by: TVD pole = TVD sensor - x * sin(f) (12.1.25) where the TVD of the sensor is at the point of closest approach to the pole. The direction to the pole in the horizontal plane, ja, relative to the survey azimuth, is given by:
    • 181 Homing-in Techniques 0.001 0.0008 0.0006 0.0004 0 960 Ftot Fxy 980 1000 1020 1040 1060 1080 1100 1120 1140 –0.0002 –0.0004 –0.0006 –0.0008 –0.001 Depth Figure 12.1.5 Field Due to a Dipole: Ftot, Fxy, Fax 0.00025 0.0002 0.00015 0.0001 Field gradient (T/m) Field (T) 0.0002 0.00005 0 950 –0.00005 DFax 1000 1050 1100 1050 –0.0001 –0.00015 –0.0002 –0.00025 Depth Figure 12.1.6 Field Due to a Dipole: DFax Fax
    • 182 Well Logging and Formation Evaluation 200 180 160 AXDIR (deg) 140 120 AXDIR HSDIR 100 80 60 40 20 0 960 980 1000 1020 1040 1060 1080 1100 1120 1140 Depth Figure 12.1.7 Field Due to a Dipole: AXdir/HSdir j a = arctan( tan(q ) * cos(a )). (12.1.26) Care should be taken to keep the sign of ja such that it is positive going clockwise from the horizontal projection of HS to the horizontal projection of the vector linking the sensor to the pole. The direction of the pole relative to north, jn, if b is the azimuth of the survey well relative to north, is given by: j n = j a + b. (12.1.27) Hence, in the horizontal plane, the relative position of the pole from the sensor position at the point of closest distance from the pole is: north: x * cos(f) * cos(j n ) (12.1.28) east: x * cos(f) * sin(j n ). (12.1.29)
    • 183 Homing-in Techniques Target well Survey well a Pole t Point of closest approach to pole d Point of closest approach to target well Figure 12.1.8 Survey Well Passing a Target Well For dipoles, the situation is more complex, and a formula can be given for only the simplified situation of the survey and target wells being parallel and the separation of the poles being small compared with the separation of the wells. A dipole may be regarded as being positive if the north pole is above the south pole, and negative if the south pole is above the north pole. The polarity of the pole may be seen by observing AXdir as the pole is passed. For a positive dipole, AXdir increases with depth, reaching 180 degrees at the point of closest approach to the dipole, then decreasing again. For a negative dipole, AXdir should decrease to zero as the dipole is passed, and then increase again. Since AXdir is 0 or 180 degrees at the near point, HSdir cannot be defined at this point. However, for a positive dipole, the direction to the target relative to HS is equal to HSdir just above the near point. For a negative dipole, it is equal to HSdir just below the near point. To find the Cartesian position of the dipole, equations 12.1.24 to 12.1.29 may be used. Distance may be found from one of the following methods. The halfwidth of Ftot is related to the distance by: x = Ftot * 0.453. (12.1.30)
    • 184 Well Logging and Formation Evaluation The width of the AXdir curve at the points where AXdir is 90 degrees is related to distance by: x = DAX dir 2. (12.1.31) The points on the Fax curve where the gradient is zero are such that the separation between the first zero and the last zero, on either side of the central zero, is related to distance by: x = DFax 6. (12.1.32) When the far field approximation no longer applies and the wells are not parallel, the situation becomes more complex, and it is not possible to use quicklook methods. However, it should be noted that at the point of closest distance from one of the monopoles forming a dipole, the field behavior of Ftot, Fxy, and Fax will be dominated by the monopole, and monopole quicklook methods may be applied. Exercise 12.1. Worked Field Example of Magnetostatic Homing In Consider the following case. A vertical exploration well has blown out, and a relief well is required for homing in and intersection of the target well. The best survey data available for the target well are as follows: Total depth: 2,200 ft TVDss Easting: 5,340 ft Northing: 6,898 ft A relief well has been drilled to pass close to the blowout well, with the survey data in Table 12.1.2. The Earth’s magnetic field has the following components: Ev = -7.75 * 10 -6 T Eh = 40.9 * 10 -6 T. Magnetic declination (the angle clockwise from grid north to magnetic north) is zero. In the relief well, a survey package consisting of three magnetometers and three accelerometers has been run. The results are shown in Table 12.1.3 (note that depths/distances are in feet and not meters).
    • 185 Homing-in Techniques Table 12.1.2 Example of well survey data TVD (z) (ft) 2073.4 2080.7 2086.7 2092.7 2098.7 2105.7 2111.7 2117.6 2123.7 2129.7 2135.8 2141.9 2148 2154.1 2161.2 2167.3 2173.5 2179.7 North (x) (ft) East (y) (ft) 6985.4 6975.9 6967.9 6959.9 6952 6941.6 6933.6 6925.6 6917.7 6909.7 6901.8 6893.9 6885.9 6878 6870.2 6862.4 6854.5 6846.7 5348.8 5348.3 5347.9 5347.2 5346.8 5346.4 5345.9 5345.5 5345.1 5345.1 5344.7 5344.3 5343.8 5343.4 5343 5342.6 5342.1 5341.7 1. Derive the components of the Earth’s field in the highside reference system. 2. Convert the raw magnetometer data to the highside reference system and subtract the Earth’s field components. 3. Determine whether any poles are present, and their polarity. 4. Use quicklook methods to estimate the shortest distance from the pole to the relief well and estimate the error in the assumed position of the target well from the relief well. 5. Model the field due to the assumed pole in the highside reference system and overlay the field so derived with the actual measured data to see how good the fit is. 6. Is there any evidence of a second pole? 12.2 ELECTROMAGNETIC HOMING IN Magnetostatic techniques have the advantage that dedicated tools are not required, but they rely on the unpredictable nature of the magnetiza-
    • 186 Well Logging and Formation Evaluation Table 12.1.3 Example of raw magnetometer tool data Depth (ft md) Ax Ay Az Bx (mT) By (mT) Bz (mT) 2700 2710 2720 2730 2740 2750 2760 2765 2770 2772.5 2775 2777.5 2780 2782.5 2785 2787.5 2790 2792.5 2795 2797.5 2800 2805 2810 2820 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 -17.81 -17.81 -17.81 -17.81 -17.81 -17.81 -18.36 -18.49 -19.86 -21.23 -23.97 -26.71 -30.14 -30.14 -29.73 -28.77 -27.4 -26.03 -23.97 -23.29 -21.92 -20.55 -19.86 -18.49 2.19 2.19 2.19 2.19 2.19 2.19 1.51 0.55 -1.51 -4.66 -8.77 -14.93 -19.73 -19.73 -12.88 -4.66 0.82 3.56 3.56 3.56 3.56 3.56 3.56 3.56 -34.76 -34.76 -34.76 -35.37 -35.37 -35.37 -36.59 -37.8 -40.24 -41.46 -42.68 -40.85 -34.15 -28.05 -22.32 -23.17 -25.61 -29.88 -31.71 -33.29 -34.15 -34.51 -34.51 -34.15 tion occurring on the target. Also, the range is typically small. Electromagnetic techniques have a greater range, but they rely on dedicated tools, and the mathematical modeling is much harder to perform. The techniques have nonetheless been used successfully in a number of blowouts. 12.2.1 Principles of Electromagnetic Homing In Continuous steel in a target well will provide a low-resistive path for any current induced into the formation. By measuring the intensity and direction of the magnetic field associated with the current on the target, the position of the target well may be determined. Hence, the main elements of an electromagnetic homing-in tool are as follows:
    • Homing-in Techniques 187 1. An electrode that injects a low-frequency current into the formation, separated by an insulated bridle (typically 100–300 ft long) from the sensor package 2. A sensor package consisting of orthogonal direct current (DC) and alternating current (AC) magnetometers. DC magnetometers are used to establish the tool orientation with respect to HS, and AC magnetometers are used to measure the magnetic fields caused by current on the target well. The AC magnetic field measured in the sensor plane is the vector sum of the magnetic fields from each element of current. In the case of an infinite line source of current, the field measured at a sensor is given by BiotSavart’s law: H = I [2 * p * dist] * [dist Ÿ Ÿ Tvec Ÿ ] (12.2.1) where: H = vector of magnetic field strength, in T I = current flowing, in amperes dist = vector linking the sensor to the line source by shortest distance, in m distŸ = unit vector along dist TvecŸ = unit vector along the line source direction. These components are illustrated in Figure 12.2.1. If the current on the target well were everywhere constant, modeling would be relatively simple, and similar algorithms to those applied in the magnetostatic methodology could be applied. Unfortunately, this is not the case, since the current is influenced by the following factors: 1. Even in a theoretical case of isotropic media, and infinite target casing of uniform thickness, predicting the current as a function of depth in the target well is a complicated mathematical process involving the use of Bessel functions and numerical integration. The field measured in the survey well is affected not only by the sensor’s proximity to the target, but by the injecting electrode’s proximity to the target casing, which both vary with depth. 2. In reality the target casing is not infinite, and often one is homing in near the shoe of a casing string, where the target current will fall to zero (yielding no magnetic field).
    • 188 Well Logging and Formation Evaluation Target well Survey well Electrode Current on target well Magnetometers in the sensor plane Field due to current Figure 12.2.1 Electromagnetic Homing-in Principles 3. The current in the target is influenced by the thickness of the casing/drillpipe (which may vary with depth) and quality of the conducting path between steel and the formation, which is also variable. In some field cases there have been attempts to overcome some of these limitations by injecting current directly into the target well at surface. While this has some advantages, it is nevertheless virtually impossible to accurately predict the current as a function of depth in the target well. During my time in research, I was involved in developing a series of Fortran programs that performed a full mathematical simulation of the tool response as a function of depth for deviated well paths, incorporating the effect of noninfinite target casing. While these programs are still available, it is beyond the scope of this book to explain the algorithms in detail. In Figure 12.2.2, a typical response of the total field strength and HSdir of the field as a survey well passes a target well are shown. Note that the direction of the field (unlike that from a monopole) varies with depth as the survey well passes the target well. This means that while
    • 189 Homing-in Techniques 360 310 300 260 Field Strength uA/m/A 250 210 Direction (deg) 350 200 160 Field strength Field direction 150 110 100 60 50 0 2600 10 2650 2700 2750 2800 2850 –40 2900 Depth (ft) Figure 12.2.2 Typical Response of an Electromagnetic Homing-in Tool modeling of the field strength may be very difficult, triangulation may also be used to help pinpoint the target well position. 12.2.2 Quicklook Interpretation Techniques Consider a simple case of a straight vertical target well being passed by a straight deviated survey well having inclination a. The behavior of HSdir as a function of depth will be given by: [ HSdir = ar cos D * cos(a ) (x 2 * sin 2 (a ) + D2 * cos 2(a )) ] (12.2.2) where: HSdir = angle from highside to the measured field in the sensor plane D = closest approach distance between survey well and target well x = measured depth in the relief well, with x = 0 at closest approach point.
    • 190 Well Logging and Formation Evaluation Table 12.2.1 Example of electromagnetic homing-in tool data Depth (ft) 2650 2655 2660 2665 2670 2675 2680 2685 2690 2695 2700 2705 2710 2715 2720 2725 2730 2735 2740 2745 2750 2755 2760 2765 2770 2775 2780 2785 2790 2795 2800 2805 2810 2815 2820 2825 2830 2835 2840 2845 2850 Field Direction (deg) Field Strength (ma/m/a) 265.46 264.67 273.54 260.41 270.72 261.09 274.32 271.95 275.94 275.1 272.5 274.87 274.72 282 275.21 272.39 279.19 279.96 280.95 284.25 282.18 282.91 287.81 286.89 285.39 289.38 295.55 308.07 341.45 25.87 52.92 65.24 72.56 78.53 82.29 83.05 86.01 98.11 94.1 98.8 109.64 52.96 60.03 53.32 57.53 55.05 59.82 59.73 64.31 58.05 57.55 60.77 64.82 69.11 64.95 75.95 87.64 82.18 90.91 91.24 97.06 107.72 116.87 127.97 150.73 183.33 211.61 262.2 304.58 305.65 320.93 281.65 214.19 172.37 139.15 111.05 94.78 71.93 63.06 61.25 56.25 48.43
    • Homing-in Techniques 191 At x = 0, HSdir is also zero, which means that the field direction is along the HS direction. When HSdir = 45 degrees and x = X45, rearranging equation 12.2.2. yields: X45 = D * cotan(a ). (12.2.3) Hence, if we plot HSdir vs. x and measure the width D over which HSdir varies from -45 to +45 degrees from its value at x = 0, we can say: D = tan(a ) * D 2 . (12.2.4) If the target well is not vertical, there will be a static shift in HSdir, and a needs to be taken as the intersection angle between the survey well and target well. Intensity data may also be used for triangulation purposes if it can be assumed that the current on the target well is approximately constant as the sensor passes the target well. It is also necessary that any background signal be removed. This is done as follows. The separation in measured depth (D) between points for which the intensity curve has fallen to half its maximum height is given by: D = ( D sin(a )) * 2 * (y + ( y 2 + 3) ) (12.2.5) where y = (2 - cos2(a)) / cos2(a) and D is the distance of closest approach. Exercise 12.2. Interpretation of Electromagnetic Homing-in Data A survey well was drilled at an inclination of 50 degrees past a vertical target well. After correction for background effects, an electromagnetic tool measured the data given in Table 12.2.1. With the quicklook methods described above, measure the distance at the point of closest approach, using both the directional and intensity data.
    • C H A P T E R 1 3 WELL DEVIATION, SURVEYING, AND GEOSTEERING 13.1 WELL DEVIATION The trajectory of a deviated well may be described in terms of its inclination, depth, and azimuth. The inclination of a well at a given depth is the angle (in degrees) between the local vertical and the tangent to the wellbore axis at that depth (Figure 13.1.1). The convention is that 0 degrees is vertical and 90 degrees is horizontal. Parts of a degree are given in decimals, rather than minutes and seconds. Gravity varies with latitude, and its direction may be influenced by local features such as mineral deposits and mountains, as well as the Earth’s rotation. Depth in boreholes is measured either along the hole itself, in which case it is referred to as measured or alonghole depths, with reference to a fixed point, or as true vertical depth (TVD) with reference to a datum. Depth references that are commonly used are as follows: • • • Derrick floor. This is the elevated deck on which the rig crew work, typically 10 m or so above ground level on a land rig and 20–30 m on an offshore rig. Also sometimes referred to as a rotary table. Kelly bushing. This is the top of the bushing, which rotates on the derrick floor (although kellys are rarely used on modern drilling rigs with topdrives) and is typically 1 ft higher than the derrick floor. Mean sea level. This is the elevation of the sea, averaging out the effect of tides or seasonal variations. Usually the topography department will establish the elevation of a land location prior to drilling. For offshore locations, the elevation of the seabed will be known. On floating rigs, a correction using tide tables will be used. 193
    • 194 Well Logging and Formation Evaluation Derrick foor Ground level or sea level Depth reference level True vertical depth TVD Survey point at md Inclination Borehole Local gravity vector Figure 13.1.1 Basic Definitions Azimuth, expressed in degrees between 0 and 360, is defined as the angle of the horizontal component of the direction of the wellbore at a particular point measured in a clockwise direction from magnetic north, grid north, or true north. These are defined: • • Magnetic north. This is the direction of the horizontal component of the Earth’s magnetic field lines at a particular point on the Earth’s surface. A magnetic compass will align itself to these lines. The angle between magnetic north (MN) and true north (TN) is defined as the magnetic declination (D). When MN lies to the west of TN, D is negative. When MN lies to the east of TN, D is positive. Typical values for D are -5 to +5 degrees. Grid north. Due to the curvature of the Earth, it is not possible to cover the surface in a regular rectangular grid pattern using meridians (i.e., lines heading north/south), although such a grid will be almost rectangular over limited areas. The central meridian in a grid will be identical to true north, but vertical grid lines to the west of center will point
    • Well Deviation, Surveying, and Geosteering • 195 west of true north in the Northern Hemisphere and east of true north in the Southern Hemisphere. Likewise, vertical grid lines to the east of the central meridian will point east of true north in the Northern Hemisphere and west of true north in the Southern Hemisphere. The so-called grid correction (G) is positive when TN is east of grid north, and negative when it is west. Typical values are -1.5 to +1.5 degrees. True north. This is the direction of the geographic north pole as defined by the axis of rotation of the Earth. The meridians, or lines of longitude, on maps point toward true north. 13.2 SURVEYING Borehole position uncertainty defines the range of actual possible positions of a particular point in a well in terms of eastings, northings, and TVD. Factors that affect borehole position uncertainty are: 1. Accuracy of measured depth determination in the well. Both drillpipe and wireline cable suffer from stretch and inaccuracies in the methods used to measure how much pipe or cable has been run into the hole. This uncertainty becomes greater with increasing depth and well deviation. For a vertical well drilled to 3,500 m, one would expect the measured depth at total depth (TD) to be known to an accuracy of roughly 2 m. For a deviated well drilled to 3,500 m TVD with a deviation at bottom of 50 degrees, this inaccuracy might rise to 5 m. 2. Frequency of survey stations. When surveying a well, it is normal to acquire “stations” at discrete depth intervals along the well. At each of these stations, the hole’s inclination and azimuth will be measured. Between stations, it is necessary to use an algorithm to determine values to be interpolated. The accuracy of the final survey will depend on the frequency of these stations and algorithm used. 3. Survey tool accuracy. Let us consider these different types of tools separately: (a) Magnetic survey tools. The accuracy of magnetic devices is limited by their intrinsic accuracy and the extent to which they are affected by magnetic interference. The strength and direction of the Earth’s field is also a factor, since in a worst-case scenario of drilling in the same direction as the Earth’s lines of flux, no azimuthal measurement would be possible. Near the poles, the Earth’s field is nearly vertical and this would be a factor. Magnetic interference may come from any metal in proximity to the tool (such as the
    • 196 Well Logging and Formation Evaluation drillstring itself, although nonmagnetic collars are always used immediately beside the tool). The tool’s intrinsic accuracy is affected by the sensitivity of the magnetometers and accelerometers (used to determine tool orientation). From published data it is estimated that lateral borehole uncertainty arising from use of properly calibrated magnetic survey tools is on the order of 14 m per 1,000 m in a vertical well and 20 m per 1,000 m in a 70-degree deviated well. (b) Gyro survey tools. These types of tools are affected by drift in the alignment of the gyro orientation during the survey. They are usually run only when casing has been set in a well, so they cannot be used for decision making during the course of drilling. From published data it is estimated that lateral borehole uncertainty arising from use of properly calibrated gyro survey tools is on the order of 1.5 m per 1,000 m in a vertical well and 8 m per 1,000 m in a 70-degree deviated well. (c) FINDS. These tools use highly accurate accelerometers and double-integrate the accelerations to determine absolute distance moved by the tool during the survey. Their accuracy is estimated at 0.5 m per 1,000 m irrespective of deviation. 13.2.1 The Effects of Borehole Position Uncertainty We need to know the positions of wells accurately for the following reasons: 1. Well safety. In the event of a well blowing out, a relief well may be required to intercept or pass close to the well. Homing in with a relief well (see Chapter 12) is made easier if the position of the target well is known accurately. Also, for well collision avoidance in densely drilled areas, the relative well positions must be known. 2. Mapping. Any geological maps are only as good as the input data. While small uncertainty in the lateral position of wells may not be critical, uncertainty in the TVD at which a certain horizon is penetrated may lead to serious errors in maps, and therefore reserves. In particular, if the depth of a fluid contact appears to be different in neighboring wells, it might lead one to make wrong judgments as to the position of faults or communication/differential depletion between wells. 3. Geosteering. When drilling horizontal wells through thin horizons, accurate measurement of TVD is of high importance.
    • Well Deviation, Surveying, and Geosteering 197 4. Pressure/gradient determination. Accurate knowledge of the TVD at which pressure measurements are made is essential for accurate determination of gradients and correction of formation/well pressures to a common datum reference. 5. Legal reasons. When drilling close to a concession or national boundary, it may be essential to avoid accidentally crossing such a boundary. There may also be implications for equity determination and unitization. Most government bodies will have minimum requirements regarding survey accuracy and maintenance of a proper database of survey data. 13.3 GEOSTEERING Geosteering is the use of information gained while drilling to make realtime decisions on the trajectory of the well. Such decisions may be essential to optimize the utility of a well. Geosteering is used (a) in high-angle deviated wells in thin formations where productivity can be achieved only if the wellbore remains in a thin permeable zone and (b) in horizontal wells where it is necessary to remain a fixed distance from either a fluid contact or an overlying tight formation, as well as during (c) drilling in close proximity to a fault where it is necessary to establish whether or not the fault is close and should be crossed and (d) drilling with a fixed orientation to natural fractures. Data that may be used in the decision-making process during geosteering concern (1) deviation; (2) cuttings, including hydrocarbon shows and gas readings; (3) transmission of LWD (logging while drilling) tools in real time, typically up/down GR (gamma ray), density, neutron, and resistivity; and (4) drilling parameters, such as losses, kicks, rate of penetration (ROP), and torque. Geosteering is often much harder in practice than anticipated, due to the following factors: • • • Tools used in the decision-making process are typically run some way behind the bit (possibly up to 30 m). Therefore, if the bit is not where you want it to be, you will often not know about it until quite a bit of formation has been penetrated. In high-angle wells, there are often problems with real-time data transmission through mud pulses arising from noise, high ROP, tool failures, battery life limitations, and bandwidth. Cuttings data may take up to 2 hours to reach surface (the “bottomsup” time). Where a turbine is used, the cuttings may be very finely
    • 198 • • • Well Logging and Formation Evaluation ground and difficult to interpret. Also, highly deviated wells are often drilled with OBM (oil-based mud), making hydrocarbon differentiation difficult. Areal variation in the formation is usually much greater than that expected from the working geological maps. It is very often the case that subseismic faults of a few meters are encountered, which cause the well to suddenly go out of the target zone. Often it is not clear whether one has exited the top or base of the target zone, so one does not know whether to drill up or down to get back in. Even where faulting is not present, there may be thinning or deterioration of reservoir properties that were not envisaged. Even where the right geosteering decisions can be made, control of deviation in the well itself may be a problem. When one is entering a thin horizon at a steep angle, it may be impossible to avoid immediately exiting the horizon on the other side. There may also be a tendency for the bit to drop or turn to the right or left, which cannot easily be controlled. In very long horizontal wells, one may be limited by the need to keep the drillpipe in tension and have sufficient weight on bit (WOB) to be able to make further progress. Where a horizontal well accidentally penetrates a water-bearing zone, there may be significant practical difficulties in preventing a large proportion of the well’s production from originating in the water zone. The possibilities of isolating certain zones in long horizontal wells are very limited. In spite of the above limitations, geosteering can be immensely valuable in drilling very highly productive wells and can make the difference between a field being economically viable or not. It may also be the case, if drilling in a permeable formation surrounded by tight formations or in a long horizontal well, that the bit will naturally follow a path of least resistance and steer itself within the most permeable layer, effectively “bouncing off” the harder layers. An example of a typical geosteered well through a thin formation is shown in Figure 13.3.1. With respect to the decisions made by the petrophysicist in the planning and execution of a geosteered well, it is worthy of consideration that while one would ideally want as many tools in the hole as possible, with both up and down measurements of all parameters, one is necessarily limited by constraints as to what the drillers are prepared to have in the toolstring (a greater number of tools and their proximity to the bit affect drillers’ ability to steer the well) and what data can be captured within the
    • Well Deviation, Surveying, and Geosteering 199 Well path 10 m 0.5 km Target horizon OWC Figure 13.3.1 Example of Geosteered Well available bandwidth (of the mud pulse telemetry system). Therefore, careful consideration should be given to which tools are most effective in determining whether or not one is in the target formation as opposed to above or below it. Bear in mind that the density/neutron tools require the toolstring to be rotating for meaningful data to be obtained, and when changing the well course it is often necessary to slide the toolstring using a turbine and bentsub. Resistivity data are generally more reliable, since they are not a statistical type of measurement. The LWD-GR devices can generally be placed closer to the bit and may be sufficient in many cases for determining whether one is exiting a target formation from above or below. If a long bit run is planned, battery life may be an issue (typical battery life is 50–100 hours), as may the downhole memory in which data, assumedly, are being recorded, which may become full after a certain number of hours. It is generally recommended to always record the data in a downhole memory in addition to pulsing to surface. To avoid making additional runs with pipe-conveyed logging at TD, it may be considered worthwhile to include tools in the toolstring set to only record downhole and not pulse to surface. When permeability or presence of fractures is a particular issue, there may be a requirement for tools (such as NMR [nuclear magnetic resonance], pressure testing, or sonic) that are not available from all the contractors. Data that are missing or of poor quality may be reacquired during
    • 200 Well Logging and Formation Evaluation a round trip, either once TD is reached or at some other point during pulling out of or running in the hole. Such decisions are typically made in conjunction with the drillers. It is recommended at the start of a geosteered well to set up a strict and rigorous system of naming data files transmitted from the rig to the office, so that there is no confusion as to whether data are pulsed or memory, and whether or not they were acquired during drilling or tripping. Distinctions also need to be made between up/down data and data for which depths have been corrected to be consistent with previous runs or known casing shoes, etc. Tool failures during geosteering are a common occurrence. It is recommended to keep accurate records of serial numbers of tools used in the hole and to check that regular calibration and maintenance are being performed. It can significantly add to the cost of a well if a toolstring has to be brought back to surface due to tool failure in the critical section of a well, and in some cases may even result in the well being lost if the openhole time becomes too great. For a geosteered well to be successful, there needs to be good communication between the petrophysicist, wellsite geologist, office geologist, and drilling department. The wellsite geologist, particularly if he has a good knowledge of the field, is usually in the best position to know which formation the well is in, but he needs the support of the petrophysicist to interpret the real-time formation evaluation data. The necessary course of action that these two decide upon needs to be fed back to the drillers so that the well trajectory is optimized. The depth offset between the up and down readings of tools can be used to establish whether the trajectory is veering structurally deeper or shallower. This is done as follows. Consider that one is drilling a low-GR sand, bounded above and below by high-GR shales. In the event that one exits the sand into the structurally shallower shale, one would expect that the up reading on the GR would respond before the down reading; similarly, if the wellbore exits to the structurally deeper shale, the down log would respond first. The offset between up and down readings, together with knowledge of the borehole size, can yield an estimate of the relative dip between the borehole and formation. Consider the example in Figure 13.3.2. If the borehole diameter is d and the offset between the up and down readings is t (measured in similar depth units as d), then the relative angle (q) between the borehole and the formation is given by: q = arctan(d t ). (13.3.1)
    • 201 Well Deviation, Surveying, and Geosteering Sand Shale d Down log GR Up log Depth t Figure 13.3.2 Example of Up/Down Response as Borehole Crosses Boundary from Above If the borehole deviation is f, the apparent formation dip (a) will be given by different formulae depending on whether the wellbore leaves by the top or bottom of the formation, and whether or not the formation dip is broadly in the same direction or opposite to that of the borehole. The appropriate formulae are: Case A. Exit by bottom, formation dip opposite to borehole: a = (f - 90 ) + q (13.3.2) Case B. Exit by top, formation dip opposite to borehole: a = (90 - f) + q (13.3.3) Case C. Exit by bottom, formation dip same direction as borehole: a = (90 - f) - q (13.3.4) Case D. Exit by top, formation dip same direction as borehole: a = (90 - f) + q These four scenarios are illustrated in Figure 13.3.3. (13.3.5)
    • 202 Well Logging and Formation Evaluation f Well a Well q f q a Case B Case A q Well f a Well f q a Case D Case C Figure 13.3.3 Four Scenarios of Wellbore Leaving a Formation If there is an angle between the borehole azimuth and formation dip direction, the true formation dip will be greater than a. In order to correct for this, the following formula should be used: tan( D ) = tan (a ) cos( g ) (13.3.6) where: D = the true formation dip a = the apparent formation dip g = the angle between the azimuth of the wellbore and the maximum dip direction. Note that in the event that one is drilling along the strike of the formation (i.e., g = 90 degrees), it is not possible to say what the true formation dip is. Exercise 13.1 Formation Dip from Up/Down Logs One is drilling an 81/2-in.-diameter hole at a deviation of 95 degrees when the reservoir is exited. The offset between the up and down readings is 2 m, with the up reading responding first.
    • Well Deviation, Surveying, and Geosteering 203 1. What is the relative dip between the borehole and formation? 2. If the direction of dip of the formation is the same as the borehole, what is the absolute formation dip? 3. Suppose that it is known that the formation dip azimuth is at an angle of 40 degrees to the borehole trajectory. What is now the true formation dip? 13.4 HORIZONTAL WELLS DRILLED ABOVE A CONTACT Often there is a requirement to drill wells a fixed distance above a water contact in order to optimize drainage. In an ideal reservoir, which is homogeneous, the contact would be at a fixed subsea depth, so in theory one would only need to keep the well at a certain TVD. In practice, contact depths may vary over a reservoir due to: • Capillary effects. If the rock quality (particularly permeability) varies, the oil/water contact (OWC) or gas/water contact (GWC) will vary, while the free water level (FWL) remains constant. • Depletion in the field. The contact may have moved due to aquifer influx or injection during production. • Depletion in neighboring fields. There may be observed an overall tilting of the contact in a certain direction due to offtake in a neighboring field affecting the aquifer. The position of the contact will typically have been determined through measurements made in nearby wells, and there may be some scatter in the interpreted contact depths due to surveying errors. Typically, this uncertainty will be on the order of 2–5 m, although it may be greater if some wells are particularly anomalous. Borehole TVD uncertainty as a result of surveying errors, coupled with uncertainty in the true contact depth, will lead to an overall uncertainty as to the distance between the well and the contact. It may therefore be necessary for the petrophysicist to assess the well’s proximity to the contact or FWL via real-time measurements during drilling. The best way to do this is by using an established saturation/height function, calibrated against core in earlier wells. Then the water saturation calculated in the horizontal well while drilling may be input to the model to back-calculate the height above the FWL. Once this is known, the OWC may be estimated by observing the entry height on the curve corresponding to the prevailing porosity and permeability.
    • 204 Well Logging and Formation Evaluation Using the equations presented in Chapter 4 and solving for h (the height above FWL) yields: h = {[( Sw - Swirr ) a]( -1 b ) (k f) } [( rho w - rho h ) * 3.281 * 0.433] (13.4.1) where: h = the height above FWL, in m Sw = log-derived saturation (fraction) Swirr = irreducible water saturation (fraction) a, b = Leverett J fitting constants k = permeability, in md f = porosity (fraction) rhow = water density, in g/cc rhoh = hydrocarbon density, in g/cc. It would be necessary to derive an empirical relationship for the height between the OWC (or GWC) and FWL and to have a log-derivable parameter (such as permeability from the poroperm relationship). In Figure 13.4.1 is a hypothetical example of a well drilled close to an FWL, using the log-derived saturation to estimate the depth of the FWL. In the figure, production from neighboring fields was known to cause a variation in the FWL. Entry height between the FWL and OWC was X00 x500 x1000 x1500 Measured depth (ft) x2000 x2500 X00 X50 Wellbore X100 TVD (ft) Predicted FWL X150 X200 Figure 13.4.1 Calculating the Position of the Free Water Level While Drilling
    • Well Deviation, Surveying, and Geosteering 205 known to be small. Note that the small-scale variation in the FWL depth is probably not real but due simply to inaccuracy/scatter in the calculated Sw. Also the fact that the FWL appears to go much deeper as the wellbore rises is almost certainly spurious and a result of inaccuracy in the saturation/height model. However, such a plot could at least be used to confirm a general dipping of the FWL along the wellbore trajectory and to establish that the well was some 50 ft above the contact. 13.5 ESTIMATING THE PRODUCTIVITY INDEX FOR LONG HORIZONTAL WELLS A technique that has often been found to give good results in estimating PI is as follows. Using the established poroperm relationship, determine k all along the horizontal wellbore. Integrating this function from the top of the objective to TD will yield the gross product k*h for the well. Make a graph of k*h vs. the PI (in b/d/psi drawdown) for a number of wells already producing in the area. Often a good correlation is found. This enables one to predict the PI while the well is being drilled, taking into account sections of the well that will be nonproducing due to poor permeability. This information may be important because the total length a well needs to be drilled may be shortened (thereby saving money) if a PI threshold has been reached above which surface facility limitations will negatively affect production. It allows comparisons to be made between wells drilled under different conditions and helps identify formation damage in wells that produce far below the established trend.
    • A P P E N D I X 1 TEST WELL 1 DATA SHEET DEPTH 616.001 616.153 616.306 616.458 616.61 616.763 616.915 617.068 617.22 617.372 617.525 617.677 617.83 617.982 618.134 618.287 618.439 618.592 618.744 618.896 619.049 619.201 619.354 619.506 619.658 619.811 619.963 620.116 620.268 620.42 620.573 620.725 620.878 621.03 621.182 621.335 621.487 GR DENSITY NEUTRON RES_DEEP RES_SHAL RES_MICR CAL DT DTS 104.638 102.528 99.254 97.172 95.056 90.259 88.342 88.537 89.109 87.462 86.037 89.761 94.643 97.146 94.166 89.936 87.886 89.367 90.908 90.69 88.706 87.438 88.737 89.631 87.758 80.711 74.763 71.779 72.245 75.435 75.969 74.33 74.164 76.349 80.955 85.094 87.045 2.663 2.654 2.634 2.648 2.664 2.639 2.613 2.638 2.664 2.683 2.671 2.641 2.652 2.651 2.637 2.629 2.636 2.643 2.639 2.653 2.666 2.677 2.676 2.659 2.636 2.632 2.616 2.616 2.65 2.649 2.657 2.65 2.636 2.627 2.64 2.714 2.742 0.129 0.131 0.128 0.124 0.118 0.11 0.107 0.107 0.11 0.112 0.111 0.113 0.112 0.109 0.105 0.104 0.105 0.11 0.117 0.125 0.125 0.117 0.112 0.108 0.105 0.103 0.105 0.106 0.101 0.094 0.091 0.101 0.114 0.125 0.131 0.136 0.147 19.841 20.287 20.183 19.741 19.241 18.752 18.612 18.705 18.581 18.396 18.327 18.215 16.986 17.263 19.81 20.603 20.175 20.086 19.978 19.667 19.177 18.944 18.952 18.971 19.672 21.349 23.062 24.493 25.87 26.751 25.661 24.482 25.18 26.063 26.335 26.703 27.91 21.747 22.23 22.155 21.774 21.26 20.65 20.471 20.519 20.272 20.045 20.01 19.898 18.492 18.692 21.735 23.073 22.72 22.624 22.564 22.313 21.781 21.532 21.597 21.569 22.196 24.062 26.109 27.926 29.99 32.015 31.747 30.546 31.35 32.496 32.811 33.219 34.409 13.946 16.053 19.915 22.498 21.176 19.21 18.568 20.157 20.543 19.703 19.32 20.959 20.651 18.447 16.25 16.234 17.906 16.82 16.566 19.505 21.885 21.834 20.027 17.955 16.724 15.974 15.909 18.621 21.6 22.549 25.408 20.963 13.826 11.819 11.81 11.769 11.782 9.277 9.259 9.241 9.205 9.169 9.151 9.097 9.062 9.008 8.936 8.918 8.918 8.918 8.936 8.936 8.936 8.954 8.972 8.972 8.918 8.882 8.882 8.864 8.828 8.811 8.811 8.828 8.882 8.972 9.115 9.097 8.972 8.972 8.972 8.972 9.008 8.972 71.991 71.613 70.83 70.163 69.291 68.946 69.323 69.294 69.41 69.671 73.536 76.975 74.788 71.832 64.523 58.767 59.427 62.615 66.49 68.539 68.012 66.741 66.741 67.38 67.902 69.199 70.222 70.338 70.859 70.975 70.831 70.042 68.771 68.134 67.232 65.81 63.387 162.778 161.923 160.152 158.646 156.674 155.893 155.299 155.402 156.159 155.317 162.649 173.815 169.102 162.418 145.893 132.831 132.792 141.077 150.34 154.973 152.672 148.766 149.845 152.037 151.621 148.701 146.252 144.264 145.679 148.337 148.447 145.551 142.787 143.077 144.663 144.809 140.982 207
    • 208 DEPTH 621.64 621.792 621.944 622.097 622.249 622.402 622.554 622.706 622.859 623.011 623.164 623.316 623.468 623.621 623.773 623.926 624.078 624.23 624.383 624.535 624.688 624.84 624.992 625.145 625.297 625.45 625.602 625.754 625.907 626.059 626.212 626.364 626.516 626.669 626.821 626.974 627.126 627.278 627.431 627.583 627.736 627.888 628.04 628.193 628.345 628.498 628.65 628.802 628.955 629.107 629.26 629.412 629.564 629.717 629.869 Well Logging and Formation Evaluation GR DENSITY NEUTRON RES_DEEP RES_SHAL RES_MICR CAL DT DTS 87.427 86.204 87.928 92.263 93.768 83.772 65.717 51.685 48.366 49.786 48.655 44.275 40.645 39.411 39.574 41.373 42.306 42.094 40.769 38.563 37.966 37.696 37.4 36.744 34.4 31.687 30.038 30.352 31.492 31.599 30.805 29.543 28.825 30.027 32.016 33.514 33.251 32.073 32.169 33.04 34.388 34.497 33.604 33.841 34.574 34.954 34.182 33.892 34.468 33.92 32.146 31.748 32.748 33.523 33.001 2.712 2.69 2.665 2.637 2.62 2.598 2.574 2.562 2.559 2.544 2.543 2.527 2.483 2.483 2.483 2.474 2.485 2.491 2.485 2.485 2.493 2.482 2.457 2.464 2.492 2.547 2.637 2.695 2.724 2.725 2.651 2.532 2.478 2.471 2.467 2.484 2.472 2.461 2.472 2.471 2.495 2.513 2.502 2.485 2.477 2.492 2.495 2.49 2.488 2.482 2.461 2.418 2.373 2.379 2.399 0.156 0.159 0.156 0.147 0.131 0.105 0.078 0.06 0.054 0.054 0.053 0.051 0.051 0.052 0.055 0.058 0.063 0.064 0.064 0.062 0.06 0.059 0.058 0.058 0.057 0.055 0.056 0.058 0.06 0.06 0.062 0.066 0.072 0.075 0.072 0.068 0.063 0.062 0.061 0.063 0.065 0.065 0.064 0.061 0.06 0.062 0.065 0.066 0.065 0.065 0.066 0.07 0.077 0.086 0.095 27.974 24.524 21.336 20.516 20.467 20.321 19.522 17.837 15.861 14.257 13.042 12.543 12.638 12.73 12.529 12.116 11.852 11.34 10.306 9.68 9.744 10.796 14.675 18.261 17.63 16.332 15.087 13.219 11.14 10.995 13.139 14.572 15.176 16.415 16.839 16.583 16.627 16.73 16.561 16.814 17.38 18.509 19.368 18.653 17.214 15.953 15.136 14.577 14.288 14.2 13.612 9.292 5.606 5.114 5.804 34.027 29.497 25.55 24.586 24.482 24.25 23.279 21.234 18.899 17.044 15.576 14.94 15.064 15.163 14.888 14.387 14.032 13.327 12.021 11.219 11.122 12.115 16.298 20.048 19.091 17.641 16.381 14.393 12.118 11.796 13.839 15.224 15.718 16.735 16.982 16.573 16.397 16.28 15.947 16.067 16.511 17.487 18.198 17.498 16.192 15.132 14.468 13.972 13.705 13.614 13.142 9.208 5.772 5.438 6.267 15.187 19.316 24.765 33.757 49.697 69.502 68.768 67.897 66.04 40.424 23.724 15.228 11.226 12.217 11.607 10.873 10.875 10.854 9.294 8.593 9.476 8.02 8.029 9.661 9.631 9.652 9.621 9.593 9.617 10.435 14.329 11.157 7.811 8.591 8.04 7.794 7.77 7.76 7.754 7.67 9.406 9.97 9.782 10.725 10.573 10.927 10.94 9.945 8.663 6.92 4.277 2.943 2.726 2.719 2.717 8.864 8.811 8.793 8.775 8.739 8.667 8.631 8.685 8.757 8.775 8.793 8.738 8.631 8.595 8.613 8.631 8.595 8.559 8.577 8.613 8.667 8.667 8.649 8.613 8.541 8.541 8.559 8.577 8.595 8.595 8.595 8.595 8.559 8.524 8.541 8.559 8.559 8.559 8.559 8.559 8.559 8.559 8.577 8.595 8.595 8.595 8.595 8.613 8.613 8.595 8.577 8.577 8.595 8.595 8.595 62.762 63.901 64.91 63.992 62.508 63.405 64.162 64.133 64.795 64.294 63.147 63.523 63.872 63.93 63.872 68.759 73.263 73.148 72.59 64.172 58.673 59.812 57.983 55.43 55.943 62.623 68.132 69.895 73.8 70.092 70.308 76.598 76.069 75.611 71.205 64.393 63.029 63.291 62.507 62.248 62.364 61.987 62.246 63.146 63.669 63.93 64.681 66.329 67.235 65.949 64.041 63.668 64.422 64.538 64.423 139.888 141.466 145.077 144.693 141.337 138.514 127.65 119.312 118.721 118.57 115.854 114.263 113.054 112.546 112.524 122.096 130.631 130.304 128.555 112.554 102.641 104.512 101.186 96.456 96.372 106.641 115.218 118.354 125.57 119.315 119.281 129.264 127.986 127.859 121.424 110.509 108.046 107.954 106.66 106.611 107.427 106.827 106.865 108.52 109.762 110.392 111.319 114.015 115.858 113.376 109.268 108.447 110.198 110.762 110.318
    • 209 Test Well 1 Data Sheet DEPTH 630.022 630.174 630.326 630.479 630.631 630.784 630.936 631.088 631.241 631.393 631.546 631.698 631.85 632.003 632.155 632.308 632.46 632.612 632.765 632.917 633.07 633.222 633.374 633.527 633.679 633.832 633.984 634.136 634.289 634.441 634.594 634.746 634.898 635.051 635.203 635.356 635.508 635.66 635.813 635.965 636.118 636.27 636.422 636.575 636.727 636.88 637.032 637.184 637.337 637.489 637.642 637.794 637.946 638.099 638.251 GR DENSITY NEUTRON RES_DEEP RES_SHAL RES_MICR CAL DT DTS 32.212 32.689 33.32 33.59 33.86 32.415 30.504 29.894 30.772 32.806 33.12 33.096 34.478 35.788 34.341 31.345 29.325 30.689 34.057 36.446 39.231 40.526 39.886 37.558 35.177 34.631 34.683 35.776 35.9 33.98 32.714 34.305 36.001 33.138 28.294 26.112 26.079 25.451 24.267 24.685 30.436 39.09 43.435 42.636 41.039 42.422 43.489 42.05 38.574 40.296 53.32 70.821 82.6 87.079 88.962 2.391 2.393 2.403 2.407 2.415 2.487 2.513 2.457 2.42 2.411 2.421 2.409 2.408 2.438 2.454 2.471 2.485 2.479 2.459 2.434 2.442 2.462 2.462 2.49 2.537 2.525 2.496 2.5 2.515 2.523 2.535 2.543 2.536 2.521 2.505 2.495 2.493 2.491 2.49 2.515 2.522 2.502 2.539 2.584 2.563 2.533 2.541 2.545 2.528 2.518 2.519 2.532 2.579 2.597 2.566 0.1 0.097 0.09 0.083 0.077 0.075 0.078 0.087 0.096 0.099 0.1 0.101 0.102 0.102 0.098 0.088 0.08 0.078 0.079 0.082 0.083 0.084 0.083 0.078 0.067 0.053 0.044 0.039 0.038 0.037 0.036 0.034 0.035 0.038 0.041 0.042 0.041 0.04 0.041 0.045 0.049 0.051 0.053 0.052 0.05 0.046 0.045 0.044 0.044 0.044 0.045 0.051 0.064 0.089 0.106 6.157 6.18 6.165 6.165 6.154 6.02 5.685 5.203 4.902 5.393 6.41 7.214 7.892 8.637 9.588 10.76 11.255 11.681 13.261 15.034 19.206 29.714 35.857 29.365 20.727 16.555 16.572 17.707 19.945 22.356 22.132 19.723 17.722 16.542 16.017 16.073 16.47 16.755 17.075 18.661 22.339 27.434 28.242 24.209 20.527 17.782 16.129 15.211 14.514 14.771 16.49 18.696 19.578 19.433 19.204 6.634 6.648 6.66 6.666 6.641 6.495 6.151 5.669 5.301 5.71 6.769 7.665 8.378 9.18 10.172 11.272 11.704 12.146 13.768 15.556 19.609 30.148 37.272 31.517 22.756 18.516 18.813 20.335 22.987 25.77 25.707 23.114 20.833 19.485 18.834 18.76 19.12 19.408 19.711 21.515 25.653 31.695 33.285 28.855 24.67 21.533 19.616 18.534 17.617 17.746 19.65 22.168 23.019 22.698 22.29 2.674 2.382 2.264 3.371 8.461 12.602 10.32 10.025 9.995 9.966 9.951 6.476 3.365 3.496 4.546 4.487 4.432 4.436 4.43 4.427 4.434 4.435 5.129 7.466 12.693 10.838 7.13 7.431 7.442 7.431 7.428 7.44 8.051 11.17 13.725 11.076 9.64 9.274 8.455 8.986 9.954 9.858 17.006 21.045 14.649 14.747 14.802 14.785 17.973 17.351 10.176 10.988 25.056 41.803 41.699 8.613 8.631 8.631 8.631 8.631 8.595 8.649 8.757 8.775 8.775 8.775 8.775 8.757 8.703 8.631 8.631 8.649 8.631 8.631 8.631 8.631 8.631 8.631 8.631 8.649 8.667 8.631 8.631 8.649 8.649 8.667 8.667 8.649 8.649 8.649 8.631 8.613 8.595 8.577 8.577 8.595 8.595 8.631 8.685 8.685 8.685 8.685 8.667 8.685 8.703 8.685 8.631 8.684 8.81 8.81 64.423 64.045 64.563 65.582 65.582 65.205 65.205 65.322 65.582 66.104 67.258 68.018 67.612 66.71 66.076 65.814 65.437 65.841 66.104 65.844 65.96 65.96 65.96 66.595 66.214 65.697 66.22 65.343 64.712 65.205 65.582 65.728 65.466 65.697 66.336 67.232 67.379 66.626 66.857 67.235 66.332 66.446 67.235 67.351 67.728 67.873 68.018 68.424 67.898 67.119 66.595 66.968 68.278 67.898 67.003 109.949 109.527 110.709 112.586 112.716 111.381 110.484 110.397 111.248 113.103 115.232 116.522 116.512 115.607 113.798 111.912 110.329 111.649 113.709 114.43 116.027 116.688 116.361 116.295 114.447 113.287 114.214 113.231 112.198 112.126 112.167 113.181 113.556 112.567 111.363 111.849 112.077 110.538 110.382 111.198 112.361 116.812 120.487 120.266 120.088 121.083 121.919 121.864 119.095 118.62 124.838 136.685 148.217 151.043 150.622
    • 210 DEPTH 638.404 638.556 638.708 638.861 639.013 639.166 639.318 639.47 639.623 639.775 639.928 640.08 640.232 640.385 640.537 640.69 640.842 640.994 641.147 641.299 641.452 641.604 641.756 641.909 642.061 642.214 642.366 642.518 642.671 642.823 642.976 643.128 643.28 643.433 643.585 643.738 643.89 644.042 644.195 644.347 644.5 644.652 644.804 644.957 645.109 645.262 645.414 645.566 645.719 645.871 646.024 646.176 646.328 646.481 646.633 Well Logging and Formation Evaluation GR DENSITY NEUTRON RES_DEEP RES_SHAL RES_MICR CAL DT DTS 90.457 86.612 73.989 56.375 41.437 36.243 37.319 38.981 41.377 43.541 42.467 38.186 33.672 31.7 31.43 31.262 31.32 31.709 31.484 30.658 30.755 31.299 30.509 28.366 27.017 27.921 28.983 29.967 31.343 32.179 31.537 29.502 28.015 26.415 26.04 27.304 29.656 30.743 30.326 29.39 29.066 30.203 31.712 31.312 29.549 27.95 27.058 27.11 27.945 28.771 27.974 27.048 26.34 25.81 25.606 2.573 2.584 2.577 2.585 2.592 2.575 2.539 2.483 2.407 2.374 2.379 2.373 2.366 2.376 2.383 2.376 2.37 2.385 2.384 2.383 2.402 2.4 2.399 2.394 2.392 2.403 2.408 2.41 2.379 2.35 2.391 2.415 2.398 2.386 2.392 2.395 2.383 2.371 2.37 2.377 2.376 2.371 2.381 2.4 2.392 2.405 2.438 2.437 2.436 2.434 2.431 2.438 2.417 2.396 2.401 0.096 0.069 0.048 0.04 0.038 0.036 0.037 0.049 0.076 0.104 0.121 0.125 0.128 0.125 0.12 0.11 0.105 0.102 0.099 0.094 0.088 0.091 0.101 0.108 0.107 0.105 0.105 0.108 0.109 0.11 0.113 0.113 0.115 0.117 0.123 0.127 0.123 0.117 0.115 0.119 0.122 0.117 0.11 0.106 0.106 0.104 0.098 0.095 0.097 0.101 0.102 0.101 0.098 0.095 0.093 17.949 16.52 15.153 11.678 8.26 6.773 6.197 5.425 4.221 3.157 2.675 2.661 2.799 2.913 2.954 2.953 2.945 2.884 2.797 2.771 2.804 2.82 2.741 2.597 2.512 2.502 2.5 2.477 2.519 2.589 2.512 2.425 2.412 2.403 2.405 2.415 2.428 2.447 2.396 2.21 1.993 1.891 1.883 1.88 1.879 1.884 1.853 1.708 1.474 1.301 1.204 1.149 1.14 1.142 1.137 20.674 19.03 17.503 13.392 9.435 7.759 7.111 6.259 4.879 3.669 3.132 3.092 3.239 3.37 3.396 3.379 3.339 3.246 3.131 3.066 3.07 3.047 2.916 2.739 2.628 2.615 2.626 2.594 2.597 2.619 2.507 2.376 2.347 2.353 2.357 2.355 2.367 2.374 2.265 2.039 1.814 1.705 1.694 1.687 1.689 1.697 1.667 1.535 1.304 1.128 1.032 0.982 0.973 0.977 0.976 38.937 38.844 32.13 22.318 15.364 12.765 11.868 11.577 11.899 12.135 5.882 2.34 1.959 1.922 2.008 2.099 2.22 2.471 2.936 2.617 2.209 2.138 1.962 2.091 2.347 2.591 2.598 2.367 2.263 2.215 2.135 2.076 2.033 1.947 1.847 1.827 1.864 1.954 2.074 2.181 2.151 2.07 2.057 2.06 2.06 2.056 2.034 1.944 1.772 1.624 1.615 1.657 1.657 1.656 1.656 8.793 8.811 8.793 8.775 8.793 8.811 8.702 8.595 8.613 8.613 8.595 8.613 8.613 8.613 8.631 8.631 8.631 8.631 8.595 8.595 8.631 8.613 8.613 8.631 8.613 8.595 8.613 8.613 8.595 8.595 8.595 8.631 8.649 8.631 8.631 8.631 8.631 8.649 8.649 8.631 8.631 8.631 8.631 8.631 8.613 8.613 8.613 8.613 8.631 8.613 8.613 8.613 8.595 8.613 8.613 66.217 65.814 65.814 65.583 65.35 64.828 64.828 64.945 65.692 66.336 66.48 66.742 66.073 65.176 64.8 64.538 64.798 65.06 64.945 66.353 68.276 69.033 69.294 68.944 68.944 69.555 70.047 70.714 71.121 71.237 71.353 71.469 70.945 69.55 68.161 67.38 66.595 65.96 65.582 64.944 64.944 64.944 66.707 70.551 72.858 76.872 75.526 69.245 67.612 68.046 68.685 69.033 68.654 68.539 68.917 149.723 146.03 136.524 124.715 116.075 112.566 113.092 114.117 116.652 118.931 118.622 116.87 113.469 110.993 110.227 109.704 110.172 110.8 110.498 112.502 115.81 117.363 117.414 115.777 115.128 116.586 117.932 119.548 120.934 121.561 121.428 120.588 118.962 115.849 113.361 112.65 112.438 111.874 111.039 109.527 109.378 109.901 113.607 119.949 122.956 128.865 126.139 115.674 113.341 114.462 115.152 123.704 122.74 122.321 122.913
    • 211 Test Well 1 Data Sheet DEPTH 646.786 646.938 647.09 647.243 647.395 647.548 647.7 647.852 648.005 648.157 648.31 648.462 648.614 648.767 648.919 649.072 649.224 649.376 649.529 649.681 649.834 649.986 650.138 650.291 650.443 650.596 650.748 650.9 651.053 651.205 651.358 651.51 651.662 651.815 651.967 652.12 652.272 652.424 652.577 652.729 652.882 653.034 653.186 653.339 653.491 653.644 653.796 653.948 654.101 654.253 654.406 654.558 654.71 654.863 655.015 GR DENSITY NEUTRON 28.219 32.685 35.118 34.207 30.807 27.907 25.065 22.921 22.451 24.11 26.754 26.965 25.919 25.238 25.709 27.049 28.549 28.627 26.675 25.398 25.165 25.374 24.821 24.692 25.506 25.068 24.615 23.581 22.976 22.774 22.925 23.777 23.509 24.581 27.002 28.932 29.491 28.8 29.471 32.795 37.643 41.577 42.294 42.609 45.926 48.929 45.373 37.001 31.543 32.914 35.819 35.336 33.285 33.013 33.582 2.4 2.387 2.383 2.395 2.409 2.407 2.405 2.403 2.398 2.394 2.378 2.374 2.373 2.374 2.377 2.36 2.353 2.362 2.378 2.391 2.418 2.46 2.485 2.468 2.417 2.406 2.421 2.426 2.425 2.426 2.431 2.459 2.529 2.578 2.533 2.478 2.474 2.472 2.47 2.473 2.479 2.46 2.473 2.527 2.554 2.531 2.496 2.499 2.504 2.505 2.5 2.472 2.47 2.495 2.507 0.09 0.089 0.089 0.089 0.092 0.101 0.112 0.118 0.12 0.119 0.12 0.118 0.117 0.118 0.119 0.116 0.111 0.104 0.102 0.104 0.105 0.104 0.101 0.101 0.102 0.099 0.097 0.098 0.096 0.092 0.086 0.082 0.085 0.091 0.098 0.102 0.102 0.103 0.103 0.103 0.096 0.09 0.088 0.088 0.089 0.086 0.085 0.088 0.09 0.093 0.1 0.105 0.105 0.098 0.093 RES_DEEP 1.113 1.095 1.154 1.151 1.048 0.994 0.979 0.987 0.989 0.95 0.846 0.801 0.837 0.849 0.839 0.833 0.841 0.838 0.835 0.838 0.831 0.868 0.988 1.02 1.041 1.116 1.119 1.128 1.126 1.119 1.148 1.295 1.612 1.741 1.633 1.604 1.609 1.612 1.628 1.786 2.421 3.552 4.243 4.325 4.337 4.33 4.353 4.154 3.004 2.116 1.868 1.768 1.744 1.736 1.737 RES_SHAL RES_MICR CAL DT DTS 0.958 0.932 0.964 0.947 0.854 0.811 0.808 0.809 0.81 0.785 0.691 0.645 0.686 0.702 0.688 0.691 0.69 0.687 0.689 0.688 0.688 0.715 0.812 0.868 0.901 0.967 0.975 0.971 0.977 0.98 1.002 1.131 1.434 1.615 1.557 1.537 1.539 1.541 1.558 1.71 2.342 3.511 4.258 4.334 4.334 4.354 4.39 4.232 3.172 2.326 2.095 1.988 1.964 1.962 1.959 1.695 1.563 1.476 1.403 1.228 1.182 1.179 1.179 1.178 1.166 1.099 1.018 1.091 1.168 1.131 1.109 1.07 1.017 1.153 1.573 1.961 1.858 1.546 1.425 1.395 1.388 1.384 1.371 1.406 2.164 2.784 2.018 1.721 1.754 1.759 1.886 1.797 2.403 5.309 5.396 3.576 3.739 4.553 5.049 5.94 4.424 2.598 2.407 2.428 2.526 2.736 3.881 5.027 3.573 2.355 8.613 8.631 8.613 8.613 8.595 8.595 8.613 8.595 8.595 8.595 8.595 8.577 8.577 8.595 8.613 8.613 8.613 8.631 8.631 8.631 8.613 8.595 8.595 8.613 8.613 8.613 8.613 8.613 8.631 8.631 8.613 8.595 8.595 8.613 8.631 8.631 8.613 8.595 8.595 8.595 8.595 8.613 8.595 8.577 8.595 8.595 8.595 8.595 8.595 8.595 8.595 8.613 8.631 8.631 8.631 68.539 68.308 68.569 68.801 69.033 68.917 68.685 68.917 69.033 68.801 68.801 68.917 68.917 69.408 69.408 69.178 69.294 69.294 69.294 69.149 69.149 69.178 69.062 68.801 68.569 68.569 68.569 68.685 68.917 69.033 69.294 69.555 69.176 68.424 68.134 68.105 67.989 67.728 67.612 67.757 67.757 67.757 68.163 69.318 70.828 70.682 70.682 70.063 68.685 69.176 70.45 71.237 71.468 70.679 69.932 123.296 124.723 126.231 126.268 125.256 123.847 122.282 121.839 121.858 122.106 123.169 123.463 123.039 123.64 123.831 123.964 124.79 124.822 124.02 123.242 123.148 123.284 122.855 122.339 122.252 122.077 121.896 121.69 121.861 121.986 122.507 123.311 122.532 121.624 122.076 122.805 122.823 122.071 122.134 123.762 125.812 127.525 128.607 130.93 135.342 136.503 134.8 129.809 124.932 126.405 130.001 131.239 130.756 129.193 128.073
    • 212 DEPTH 655.168 655.32 655.472 655.625 655.777 655.93 656.082 656.234 656.387 656.539 656.692 656.844 656.996 657.149 657.301 657.454 657.606 657.758 657.911 658.063 658.216 658.368 658.52 658.673 658.825 658.978 659.13 659.282 659.435 659.587 659.74 659.892 660.044 660.197 660.349 660.502 660.654 660.806 660.959 661.111 661.264 661.416 661.568 661.721 661.873 662.026 662.178 662.33 662.483 662.635 662.788 662.94 663.092 663.245 663.397 Well Logging and Formation Evaluation GR DENSITY NEUTRON RES_DEEP RES_SHAL 33.535 39.277 54.047 72.013 83.311 86.014 83.945 80.051 78.528 80.412 83.821 86.215 89.724 93.307 94.173 91.692 88.118 84.661 80.708 76.543 72.806 71.58 72.166 73.663 76.248 78.858 81.809 86.383 89.934 89.916 85.813 82.977 84.186 83.179 75.267 62.968 52.38 47.784 46.37 46.996 48.843 49.156 46.855 42.049 35.986 32.841 34.566 40.057 46.354 54.532 61.468 59.526 50.97 43.547 42.758 2.496 2.493 2.499 2.503 2.53 2.579 2.627 2.638 2.638 2.652 2.648 2.651 2.49 2.355 2.388 2.377 2.346 2.328 2.314 2.298 2.295 2.254 2.144 2.102 2.152 2.295 2.403 2.394 2.395 2.423 2.474 2.512 2.504 2.474 2.472 2.506 2.559 2.592 2.592 2.577 2.568 2.558 2.562 2.558 2.601 2.711 2.747 2.707 2.658 2.664 2.715 2.753 2.75 2.729 2.761 0.085 0.083 0.097 0.124 0.149 0.152 0.139 0.13 0.139 0.16 0.175 0.19 0.234 0.291 0.317 0.299 0.275 0.264 0.269 0.296 0.339 0.378 0.394 0.381 0.367 0.378 0.397 0.376 0.314 0.254 0.219 0.213 0.219 0.195 0.133 0.074 0.053 0.056 0.061 0.065 0.066 0.065 0.06 0.057 0.056 0.056 0.056 0.058 0.075 0.1 0.105 0.084 0.059 0.051 0.065 1.743 1.739 1.725 1.984 3.903 9.3 12.635 12.255 12.233 12.224 12.225 12.23 12.224 12.023 8.773 5.573 4.922 5.206 5.345 5.353 5.35 5.337 5.347 5.559 6.201 6.487 5.601 4.856 4.842 5.039 5.433 6.689 9.4 12.058 13.282 13.825 13.869 13.871 13.891 13.995 13.831 13.737 20.455 40.566 55.781 56.167 55.513 55.572 52.537 32.124 16.039 13.122 16.073 20.442 25.074 1.958 1.967 1.963 2.224 4.189 9.912 13.841 13.462 13.413 13.418 13.446 13.449 13.429 13.307 10.07 6.781 6.21 6.613 6.783 6.77 6.769 6.79 6.816 7.028 7.7 8.075 7.155 6.265 6.265 6.561 7.035 8.583 11.981 15.27 16.889 17.696 17.757 17.743 17.746 17.853 17.502 16.701 23.181 44.734 62.045 62.409 61.573 61.559 57.481 34.036 16.857 14.082 17.432 22.302 27.414 RES_MICR 2.31 2.517 2.634 2.819 2.87 2.825 4.334 11.277 12.922 3.005 0.945 0.914 1.148 1.505 1.682 1.483 0.963 0.749 0.476 0.3 0.311 0.938 1.833 0.747 0.38 0.363 0.478 0.585 0.533 1.704 5.423 6.754 9.541 11.043 12.545 6.216 7.26 18.815 19.612 15.226 12.74 11.684 18.541 24.387 22.147 41.052 51.311 31.761 27.752 32.846 26.91 14.221 12.02 53.289 136.415 CAL 8.649 8.649 8.649 8.649 8.649 8.649 8.685 8.739 8.739 8.739 8.757 8.81 9.024 9.205 9.626 10.226 10.407 10.425 10.425 10.443 10.443 10.443 10.389 10.495 11.098 10.796 9.921 9.725 9.689 9.49 9.277 9.277 9.241 9.115 9.062 8.99 8.828 8.703 8.685 8.703 8.703 8.685 8.685 8.685 8.685 8.685 8.685 8.703 8.721 8.739 8.792 8.846 8.846 8.846 8.864 DT DTS 69.932 69.932 69.815 68.944 72.042 74.649 72.594 71.468 72.473 69.798 66.22 66.741 65.297 64.559 65.088 63.897 63.234 63.897 65.46 66.481 66.858 67.003 67.003 67.119 66.067 61.76 58.653 58.624 60.027 63.911 70.366 74.687 76.439 73.479 65.302 61.319 59.683 60.192 65.595 68.241 65.594 63.287 61.072 62.084 67.294 69.024 66.377 66.093 65.41 62.854 61.314 60.45 60.334 61.849 62.501 128.053 130.578 137.326 145.04 158.484 166.037 160.11 155.176 156.406 151.769 145.979 148.57 147.473 145.975 147.169 144.478 141.868 141.34 142.505 142.354 141.084 140.721 141.04 142.108 141.303 133.461 128.25 130.589 135.689 144.459 156.381 164.083 168.755 161.56 139.128 124.637 116.692 115.773 125.539 130.89 126.64 122.32 117.083 117.039 124.248 126.097 121.967 123.741 125.177 123.85 123.932 121.31 117.371 117.204 118.115
    • 213 Test Well 1 Data Sheet DEPTH 663.55 663.702 663.854 664.007 664.159 664.312 664.464 664.616 664.769 664.921 665.074 665.226 665.378 665.531 665.683 665.836 665.988 666.14 666.293 666.445 666.598 666.75 666.902 667.055 667.207 667.36 667.512 667.664 667.817 667.969 668.122 668.274 668.426 668.579 668.731 668.884 669.036 669.188 669.341 669.493 669.646 669.798 669.95 670.103 670.255 670.408 670.56 670.712 670.865 671.017 671.17 671.322 671.474 671.627 671.779 GR DENSITY NEUTRON RES_DEEP RES_SHAL RES_MICR CAL DT DTS 42.978 46.432 60.562 81.583 101.404 110.747 112.733 114.752 114.34 112.368 104.559 94.888 88.545 89.031 96.937 105.061 108.379 104.656 98.318 91.383 81.948 70.928 63.338 62.146 61.512 59.188 58.35 59.439 60.28 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 2.822 2.73 2.497 2.333 2.301 2.361 2.514 2.627 2.652 2.649 2.583 2.457 2.353 2.373 2.344 2.293 2.37 2.463 2.511 2.541 2.567 2.601 2.586 2.541 2.537 2.546 2.543 2.52 2.544 2.671 2.769 2.683 2.584 2.605 2.592 2.563 2.565 2.544 2.529 2.517 2.538 2.595 2.59 2.562 2.569 2.563 2.544 2.552 2.561 2.558 2.583 2.633 2.634 2.571 2.536 0.1 0.139 0.156 0.17 0.211 0.264 0.288 0.269 0.248 0.235 0.203 0.162 0.145 0.176 0.239 0.301 0.321 0.285 0.215 0.157 0.126 0.11 0.104 0.102 0.105 0.117 0.131 0.135 0.124 0.117 0.136 0.174 0.197 0.186 0.159 0.135 0.124 0.12 0.125 0.131 0.135 0.128 0.118 0.114 0.12 0.133 0.138 0.137 0.125 0.109 0.089 0.076 0.078 0.098 0.127 32.62 24.49 11.764 8.069 6.919 6.284 6.074 5.976 5.916 5.862 6.358 7.981 9.758 9.938 8.964 8.554 8.763 8.956 9.773 9.235 6.574 4.807 4 3.517 3.213 3.137 3.168 3.1 2.611 2.611 5.469 8.831 7.277 6.656 6.632 6.6 6.148 5.07 3.992 3.663 4.01 4.284 4.28 4.275 4.297 4.297 4.326 4.41 4.703 5.334 5.282 4.895 4.891 4.877 4.891 35.294 26.146 12.709 8.888 7.632 6.907 6.652 6.543 6.44 6.332 6.785 8.348 9.906 9.8 8.745 8.269 8.333 8.376 9.031 8.602 6.297 4.728 3.993 3.541 3.28 3.182 3.171 3.102 2.594 2.524 5.101 8.284 6.976 6.394 6.397 6.395 5.985 4.945 3.893 3.578 3.909 4.168 4.185 4.187 4.175 4.189 4.234 4.351 4.686 5.482 5.614 5.212 5.22 5.221 5.232 70.827 27.421 10.735 8.285 6.995 3.741 1.031 1.07 3.66 5.731 7.593 3.98 3.559 8.346 2.448 0.454 1.195 7.877 13.546 9.053 5.202 4.334 3.873 3.967 3.924 2.776 2.073 1.765 2.14 3.213 7.037 19.87 16.911 10.44 4.42 3.991 5.524 3.123 3.113 3.732 4.564 4.355 4.987 7.207 8.617 11.226 10.479 5.615 4.608 6.349 15.234 26.585 21.736 12.807 6.599 8.864 8.9 9.008 9.186 9.42 9.564 9.345 9.08 9.044 9.062 9.044 8.99 9.026 9.097 9.187 9.223 9.277 9.259 9.133 9.098 9.098 9.151 9.133 9.08 9.098 9.062 9.026 9.044 9.044 8.99 8.918 8.9 8.972 9.044 9.151 9.187 9.097 9.062 9.026 8.972 8.918 8.9 8.918 8.918 8.918 8.918 8.918 8.918 8.9 8.864 8.828 8.793 8.81 8.864 8.882 61.233 61.233 61.087 60.07 59.551 60.187 61.49 62.132 62.248 63.143 64.449 65.609 66.998 67.989 67.611 67.611 67.989 67.989 68.279 69.059 70.564 71.96 72.222 71.321 70.046 69.178 68.917 68.801 68.685 68.569 68.569 68.278 67.612 67.119 66.626 66.481 66.742 67.235 67.351 67.003 66.771 67.262 67.757 67.757 68.393 68.422 67.757 67.118 67.147 68.763 69.291 67.898 66.974 64.498 61.46 115.807 117.215 123.058 131.231 134.651 136.088 139.034 140.486 140.748 142.773 145.724 148.349 150.577 153.113 152.874 152.874 153.73 153.73 154.385 156.15 154.383 150.752 147.002 144.522 141.603 138.651 137.703 138.024 138.22 137.844 137.844 137.258 135.919 134.929 133.937 133.647 134.171 135.161 135.395 134.696 134.23 135.215 136.211 136.211 137.49 137.549 136.211 134.926 134.984 138.233 139.296 136.494 134.638 129.66 123.553
    • 214 DEPTH 671.932 672.084 672.236 672.389 672.541 672.694 672.846 672.998 673.151 673.303 673.456 673.608 673.76 673.913 674.065 674.218 674.37 674.522 674.675 674.827 674.98 Well Logging and Formation Evaluation GR 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 DENSITY NEUTRON RES_DEEP RES_SHAL 2.566 2.586 2.563 2.55 2.541 2.538 2.554 2.556 2.578 2.592 2.584 2.59 2.624 2.641 2.611 2.594 2.593 2.599 2.603 2.6 2.566 0.142 0.141 0.137 0.132 0.126 0.122 0.133 0.151 0.166 0.178 0.193 0.213 0.223 0.211 0.175 0.126 0.099 0.103 0.122 0.134 0.138 4.958 5.413 6.309 6.646 6.365 6.251 6.239 6.058 7.214 10.915 13.625 14.415 15.067 14.945 14.777 14.712 14.692 14.662 13.366 9.165 10.125 5.301 5.775 6.777 7.24 6.966 6.862 6.872 6.642 7.741 11.557 14.51 15.385 16.062 15.978 15.835 15.741 15.697 15.687 14.299 9.891 10.82 RES_MICR 5.109 5.075 7.113 8.497 9.771 9.529 6.769 6.641 6.73 8.908 12.266 12.964 -999.25 -999.25 -999.25 -999.25 -999.25 -999.25 -999.25 -999.25 -999.25 CAL DT DTS 8.882 8.864 8.846 8.828 8.828 8.828 8.811 8.828 8.828 8.828 8.828 8.828 8.846 8.846 8.846 8.828 8.811 8.846 8.9 8.9 8.882 61.575 62.48 62.248 62.392 62.886 63.522 65.195 67.771 69.957 70.715 70.715 70.57 70.425 70.309 70.453 70.715 70.715 70.308 69.787 69.555 69.323 123.783 125.603 125.136 125.426 126.419 127.697 131.061 136.24 140.634 142.158 142.158 141.866 141.574 141.341 141.632 142.158 142.158 141.339 140.292 139.826 139.359
    • A P P E N D I X 2 ADDITIONAL DATA FOR FULL EVALUATION Formation Pressure Data Depth M (TVD) Fpress (psia) 624 630 636 642 646 649 652.5 662 5177.00 5184.30 5191.40 5198.60 5203.60 5208.10 5213.00 Tight Core Description Depth (m) Lithology 616–622.5 622.5–625 625–626.5 626.5–637.5 637.5–639 639–652 652–655.5 655.5–660 660–662 662–664 664–675 Shale Sandstone Limestone Sandstone Shale Sandstone Silty sandstone Shale Sandstone Limestone Shale 215
    • 216 Well Logging and Formation Evaluation Conventional Core Analysis Depth Plug Porosity (%) 620 622 624 626 628 630 632 634 636 638 640 642 2.0 2.0 11.05 1.0 9.5 15.6 15.0 7.5 10.5 6.0 17.9 15.6 Horizontal permeability (md) Grain Density (g/cc) 0.01 0.02 22.0 0.03 10.5 135.6 120.0 11.0 15.3 0.80 350 130 2.675 2.675 2.665 2.720 2.665 2.662 2.658 2.674 2.666 2.660 2.651 2.649 SCAL ANALYSES Porosity as Fraction at Overburden Pressure (psi) 50 500 1500 2000 2500 4500 6000 Sample 1 Sample 2 Sample 3 Sample 4 0.080 0.078 0.077 0.076 0.076 0.073 0.072 0.120 0.117 0.115 0.114 0.113 0.110 0.107 0.140 0.137 0.135 0.133 0.132 0.127 0.125 0.170 0.167 0.163 0.161 0.161 0.155 0.153 Brine Permeability in Measured Depth at Overburden Pressure (psi) Sample 1 Sample 2 Sample 3 Sample 4 50 500 1500 2000 2500 4500 6000 2 1.757685 1.492423 1.395571 1.318647 1.061196 0.922045 12 10.58051 9.02435 8.429014 7.909027 6.325671 5.504435 60 52.54248 45.0698 42.12812 39.50155 31.62694 27.61859 540 474.8736 402.4928 377.8645 355.9868 286.3587 248.4957
    • 217 Additional Data for Full Evaluation FRF Measurement Data Porosity FRF 0.1 0.15 0.17 0.08 0.14 0.13 89.27854 44.01849 33.36747 133.3982 43.31214 58.2223 Resistivity Index Data Plug 1 (por = 17%) Plug 2 (por = 15%) Plug 3 (por = 13%) Sw I = (Rt/Ro) Sw I Sw I 1 0.8 0.6 0.4 0.35 0.3 0.28 1 1.61592 2.955611 8.05591 10.15069 12.88598 15.20306 1 0.8 0.6 0.5 0.4 1 1.541192 2.660774 3.783241 6.397332 1 0.9 0.7 0.6 0.55 0.5 1 1.297253 2.353045 3.247888 3.986601 5.242788 Air/Brine Capillary Pressure Curves Pc (psi) phi 0.131 0.058 0.032 0.179 K 3.000 10.000 25.000 50.000 125.000 200.000 67 3.7 3.8 278 0.861 0.963 0.942 0.73 0.617 0.874 0.85 0.43 0.388 0.782 0.739 0.282 0.29 0.673 0.638 0.214 0.239 0.597 0.572 0.176 0.216 0.525 0.512 0.144
    • A P P E N D I X 3 SOLUTIONS TO EXERCISES CHAPTER 2: QUICKLOOK LOG INTERPRETATION Exercise 2.1: Quicklook Exercise 140 8 CAL INCH 18 0 CR API 100 DT US/FT 40 NEUTRON .45 FRCT –.15 .2 RES_DEEP OHMM 2000 DENSITY G/CC 2.95 .2 RES_SHAL OHMM 2000 “DEPTH” 1.95 625 650 675 218
    • 219 Solutions to Exercises 1. 2. 3. 4. 7. 9. GRsa = 20, GRsh = 90 Calculate Vsh according to Vsh = (GR - GRsa)/(GRsh - GRsa) OWC at 646 m Assume a fluid density of 1.0 g/cc in the water leg and 0.9 in the oil leg. Rw = 0.02 ohm OWC at 646 m 11. Zone Top (m) Base (m) Gross (m) Net (m) Av. Por. (m) Sw Zone 1 Zone 2 oil Zone 2 water Zone 3 water 616 622.5 646 655.5 622.5 646 655.5 675 6.5 23.5 9.5 19.5 0 21.5 9.5 1.52 0.108 0.124 0.05 0.509 0.937 0.767 12. Suggested stations for pretest measurements are: 1. 624 m reference density log 2. 630 3. 636 4. 642 5. 646 6. 649 7. 652.5 8. 662 If the oil type is unknown, it is recommended to take a sample at 630 m using a pump-out module to avoid contamination with WBM.
    • 220 Well Logging and Formation Evaluation Exercise 2.2: Using Pressure Data The following plot may be generated: 5100 600 5150 Pressure (psia) 5200 5250 5300 610 Depth (m tvdss) 620 630 640 650 660 670 Form Press OWC-646 FWL-646 oil (0.85 g/cc) water (1.02 g/cc)
    • 221 Solutions to Exercises 1. From this the FWL is picked at 646 m. Note that because the sand is of reasonable quality no significant gap between the FWL and OWC would be expected. The oil density is 0.85 g/cc. The water density is 1.02 g/cc 2. Oil bearing 3. In order to avoid early water production it is recommended to use the tight streak at 638 m to help avoid early water breakthrough. The following interval is therefore proposed: 622.5–638 m (reference density log). The final evaluated logs should look like this: DT 140 CAL 8 INCH API 40 NEUTRON 18 CR 0 US/FT .45 1.95 ZONE1 ZONE2_0IL 625 RES_DEEP –.15 .2 DENSITY “DEPTH” 100 FRCT G/CC SHPOR OHMM 2000 0 RES_SHAL 2.95 .2 OHMM 2000 0 0 0 ZONE2_WATER 650 ZONE3 0 675 M .25 POR M .25
    • 222 Well Logging and Formation Evaluation CHAPTER 3: FULL INTERPRETATION Exercise 3.1: Full Evaluation of the Test1 Well POR/PORatm From the core data it may be seen that the current cutoff point at 50% Vsh is appropriate. Calibrating the log porosity against the core: From the plots of porosity and permeability against isostatic stress the following conversions are estimated: 1.000 0.980 0.960 0.940 0.920 0.900 0.880 0.860 0.840 0.820 0.800 plug1 plug2 plug3 plug4 model (0.95) 0 1000 2000 3000 4000 5000 6000 Pisostatic PERM/PERMatm fin situ = fair * 0.95 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 plug1 plug2 plug3 plug4 model (0.7) 0 1000 2000 3000 Pisostatic k in-situ = k air * 0.7 4000 5000 6000
    • 223 Solutions to Exercises The poroperm relationship is: Core Permeability (In Situ) 1000 Log(y) = –1.93 + 27.4x 100 10 1 0.1 0.05 0.10 0.15 .25 0.20 Core Porosity (In Situ) k = 10 Ÿ( -1.93 + 27.4 * f) Making a histogram of the core grain densities (excluding the limestone plug) yielded an average grain density of 2.66 g/cc. 4 3 2 Frequency 1 0 2.645-2.65 2.65-2.655 2.655-2.66 2.66-2.665 2.665-2.67 2.67-2.675 2.675-2.68 Grain Density (g/cc)
    • 224 Well Logging and Formation Evaluation Plotting the in-situ corrected porosities vs the density yields: 1.0 0.8 Por = (2.66-Den)/(2.66 – 0.766) Por (frct) 0.6 0.4 0.2 0 1.95 2.15 2.35 2.55 2.75 Den (g/cc) Grain density = 2.66 g cc (fixed by core data ) Fluid density in oil leg = 0.7663 g cc Plotting F vs. f yields an m value of 1.9 Derivation of m 1000 FRF 100 10 0.01 0.1 Por 1 1 FRF Model (m=1.9) 2.95
    • 225 Solutions to Exercises Combining the I vs. Sw measurements yields an n value of 2.1. n from core measurements 1000 Plug1 Plug2 10 Plug3 Model (n=2.1) I 100 1 1 0.1 Sw 0.1 1.0 Rt (ohmm) 10 100 1000 Changing the porosities and m required the Pickett plot to be reperformed, yielding a revised value for Rw of 0.025 ohmm. 0.01 Rw=0.025 m=1.9 n=2.1 0.01 0.10 Por (frst) 1.0
    • 226 Well Logging and Formation Evaluation Revised sums and averages are: Top (m) Gross (m) Net (m) Av. Por. 6.5 23.5 9.5 0 21.5 9.5 0.105 0.538 40.5 0.13 0.922 188.3 655.5 675 Zone 1 Zone 2 oil Zone 2 water Zone 3 water Base (m) 616 622.5 622.5 646 646 655.5 Zone 19.5 Sw karith 1.52 0.051 0.698 kgeom kharm 16.5 51.5 0.336 5.848 5.33 0.023 0.435 The new EHC for the oil zone is 1.043 m. This compares to 1.14 m from the quicklook interpretation. The difference is -9%, and is attributable mainly to the revised value of n used. The final evaluation of the logs looked like this: CPDR_IS 0 NEUTRON CAL 8 INCH 18 API FRCT –.15 “DEPTH” CR 0 .45 100 1.95 G/CC 2.95 .2 DENSITY 39.37¨/1000M RES_DEEP ZONE1 620 ZONE2_0IL 630 640 ZONE2_WATER 650 ZONE3 660 670 OHMM 2000 0 OHMM 2000 .25 M PERM .25 .01 POR RES_SHAL .2 M SHPOR 0 M M 1000 CPERM_IS .25 .01 M 1000
    • 227 Solutions to Exercises CHAPTER 4: SATURATION/HEIGHT ANALYSIS Exercise 4.1: Core-Derived J Function A value of Swirr of 0.05 has been assumed. Fitting Swr to J: 1 Swr Core data Model 0.1 1 10 100 J This yields a = 0.45 , b = -0.3. The full function is: Sw = 0.05 + 0.45 * JŸ( -0.3) J = (1.02 - 0.85) * 0.433 * 3.281 * (h) * (k f) 26 h = (646 - depth ) Where h is in m, k in md, f as fraction. Using the core derived J function actually resulted in a slight drop (2%) in the equivalent hydrocarbon column. This indicates that thin beds are not much of an issue in this sand, as is evidenced by the logs. Also, the deep resisitivity is a good estimate of Rt.
    • 228 Well Logging and Formation Evaluation Exercise 4.2: Log-derived J function Swr 1.0 A value of Swirr of 0.05 has been assumed. Fitting Swr to J: 0.01 0.1 Log (Swr) = 0.354 – 0.458*log (J) 0.1 1.0 J 10 This yields a = 0.354, b = –0.458. The full function is: Sw = 0.05 + 0.354 * JŸ( -0.458) J = (1.02 - 0.85) * 0.433 * 3.281 * (h) * (k f) 26 h = (646 - depth ) Where h is in m, k in md, f as fraction. There is virtually no difference between the core and log derived functions, so it does not really matter which is used. I would nevertheless recommend to use the core-derived function, since this can be more easily updated if further core data becomes available.
    • 229 Solutions to Exercises Some generic sat/ht curves for a range of porosities are shown below: 200 Hafwl (m) 250.000 180 Pc (psi, res cond) 160 porosity 140 200.000 120 150.000 100 80 100.000 60 40 0.05 0.1 0.12 0.15 0.18 50.000 20 0 0.000 0.0 0.2 0.4 0.6 0.8 1.0 Sw CHAPTER 5: ADVANCED LOG INTERPRETATION TECHNIQUES Exercise 5.1. Shaly Sand Analysis The BQv relationship is as follows: Cwa (1/ohmm) Cwa vs 1/por 200 180 160 140 120 100 80 60 40 20 0 0 5 10 15 1/por BQv = (0.18 - f) (0.02 * f) 20 25
    • 230 Well Logging and Formation Evaluation Using this function to convert the core F values to F* yields m* as follows: 1000 100 F* F* m*=2.2 10 0.01 0.1 1 1 Por m* = 2.2 Likewise, converting I to I* yields n* as follows: 100 I* 10 1 1 0.1 Sw n* = 2.3 I* n*=2.3
    • 231 Solutions to Exercises The new sums and averages are: Zone Top (m) Base (m) Gross (m) Net Av. Por. Sw Zone 2 oil 622.5 646 23.5 21.5 0.105 0.480 Using the Waxman-Smits yields slightly more oil (+8%) than Archie, but the effect is small. It is still recommended to use the core-derived J function for STOIIP determination. Exercise 5.2: Fuzzy Logic The distributions are as follows: 0.06 Relative Probability 0.05 0.04 0.03 Sand Shale 0.02 0.01 0 0 50 100 150 –0.01 GR (API) Since this is only a two-variable system, the net/gross will be the same as if a single cutoff was used at the point at which the two distributions intersect (54 API). This contrasts with the previous Vsh cutoff used, equivalent to 55 API. The effect of using the fuzzy logic is therefore very little different from applying a normal Vsh cutoff.
    • 232 Well Logging and Formation Evaluation Exercise 5.3: Thin Beds The Thomas-Steiber fd/fn plot is as follows: 0.4 0.35 Quartz completely replaced by shale 0.3 0.25 Clean sand PHId 0.2 Dispersed Laminated 0.15 Structural 0.1 100% shale Logs 0.05 0 –0.05 0 0.1 0.2 0.3 0.4 0.5 Sand with 100% shale porefill –0.1 –0.15 PHIn The plot shows that the shale is primarily dispersed, rather than structural or laminated. Hence we can have confidence that our final evaluation is largely correct. The Thomas-Steiber fd/Vp plot and fn/Vp plot are as follows: 0.4 0.3 PORd 0.2 0.1 0 –0.1 –0.2 4000 4200 4400 4600 4800 Vp (m/s) 5000 5200 5400
    • 233 Solutions to Exercises 0.5 PORn 0.4 0.3 0.2 0.1 0 4000 4200 4400 4600 4800 5000 5200 5400 Vp (m/s) These show considerable scatter, and I do not believe that they show anything useful in this case. Exercise 5.4: Thermal Decay Neutron Example The value of Sw is 0.28. If Sshale is increased to 30, the value of Sw becomes 0.20. If it is lowered to 20, Sw becomes 0.35. Therefore the uncertainty in Sw resulting from the uncertainty in Sshale is ±8 s.u. Exercise 5.5: Error Analysis Based on the Monte-Carlo analyses, the following ranges are proposed for the average properties for this well: The exercise was performed on zone 2 only, with the saturation analysis performed only over the oil-bearing part. Parameter Mean s.d. Suggested Error Net/gross Porosity Sw (Archie) Sw (cap curve) 0.944 0.108 0.538 0.532 0.0049 0.003 0.15 0.014 ±0.01 ±0.006 ±0.3 ±0.03
    • 234 Well Logging and Formation Evaluation CHAPTER 6: INTEGRATION WITH SEISMIC Exercise 6.1: Synthetic Seismogram The synthetic seismogram should look something like this: No effects due to the OWC are detectable. Note that in this field there is very little sonic contrast between the sands and shales. Exercise 6.2: Fluid Replacement Modelling The following plot compares AI(oil) with AI(water). As expected, the AI in the water-bearing case is slightly higher than in the oil-bearing case, but the effect is moderate because of the relatively low porosities involved.
    • 235 Solutions to Exercises AI 8.E+06 1.E+07 2.E+07 615 625 Depth 635 Oil bearing 645 Water-bearing 655 665 675 Exercise 6.3: Acoustic Impedance Modelling The AI distributions for the shales, oil case, and water case are as follows: 8.00E-07 7.00E-07 Probability 6.00E-07 5.00E-07 Shales 4.00E-07 Oil sands 3.00E-07 Water sands 2.00E-07 1.00E-07 0.00E+00 8.00E+06 1.00E+07 1.20E+07 1.40E+07 AI There is virtually no AI contrast between the shales and sands, irrespective of porefill. It may be seen that even if the water sands might be distinguishable from shale (ignoring resolution effects), the effect of the formation becoming oil bearing is that it would seem to disappear to within the shale distribution.
    • 236 Well Logging and Formation Evaluation CHAPTER 7: ROCK MECHANICS ISSUES Exercise 7.1: Net Effective Stress Ignoring the poroelastic constant the uniaxial stress should be: 12, 000 psi - 12, 000 * 0.435 = 6780 psi. The conversion from uniaxial to isostatic is given by: Isostatic uniaxial = (1 + 0.35) (3 * (1 - 0.35)) = 0.692 Hence the experiments should be performed at 0.692*6780 = 4693 psi. If the poroelastic constant is 0.85 the true effective stress is given by: 12, 000 - 0.85 * (12, 000 * 0.435) = 7563 psi. The experiments should be performed at 0.692*7563 = 5234 psi. CHAPTER 8: VALUE OF INFORMATION Exercise 8.1: Decision Tree Analysis The following decision tree should be used to establish the EMV of running the tool: Yes Do nothing Zone flushed? Yes No Recomplete: +$M1.4 Yes –$M0.02 Tool reliable? Yes No TDN? No Zone flushed? Recomplete: –$M2.2 No Do nothing Do nothing
    • 237 Solutions to Exercises The DEMV (in $M) is given by: DEMV = -0.02 + 0.7 * 0.5 * [20 * (180, 000 - 60, 000 ) - 1] + 0.3 * 0.5 * [20 * ( -60, 000 ) - 1] = $0.14 M The DEMV expressed as a function of the reliability, R, is given by: DEMV( R) = -0.02 + R * 0.5 * (1.4) + (1 - R) * 0.5 * ( -2.2 ) DEMV The EMV becomes zero at a value of R of 0.62. So unless the tool will give the right answer at least 62% of the time, it is not worthwhile to run it. 0.8 0.6 0.4 0.2 0.0 –0.2 –0.4 –0.6 –0.8 –1.0 –1.2 –1.4 0.000 0.200 0.400 0.600 0.800 1.000 Reliability If there is an estimated 70% chance that the zone is not flushed, the DEMV becomes: DEMV( R) = -0.02 + R * 0.7 * (1.4) + (1 - R) * 0.3 * ( -2.2 ) For R = 0.7, the DEMV becomes $0.468 M; the R required for 0 EMV becomes 0.42 If there is an estimated 30% chance that the zone is not flushed: DEMV( R) = -0.02 + R * 0.3 * (1.4) + (1 - R) * 0.7 * ( -2.2 )
    • 238 Well Logging and Formation Evaluation For R = 0.7, the DEMV becomes $-0.188 M; the R required for 0 DEMV becomes 0.80. If there is a 50% chance of the zone being flushed, and the tool predicts the zone to be unflushed with 70% reliability but is only reliable 60% of the time in predicting a flushed case. When the zone is flushed, the EMV becomes: DEMV( R) = -0.02 + 0.70 * 0.5 * (1.4) + ( 0.4) * 0.5 * ( -2.2) = $0.03 M CHAPTER 9: EQUITY DETERMINATION Exercise 9.1: Optimizing Equity Note that the test2 well has significantly better sand quality than test1. It is also updip and does not see the OWC. You would obviously maximize your equity if the determination were to be done on the basis of GBV or NPV. However, you have no technical case to support this, so you must expect that the final basis will be on HCPV. Since neither well encountered gas, there is no gas conversion factor to consider in this example. The test2 well was drilled with OBM and it appears that the induction tool is saturating, giving unrealistically high values of Rt that in turn lead to very low Archie water saturations. On this basis you should definitely push for a saturation/height based model for the saturations: Within the range of reasonable Vsh cutoff values that could be chosen there is no dependence between Vsh cutoff and equity. This is hardly surprising since the sands are very well defined. Increasing matrix and fluid densities, both of which have the effect of increasing the porosity, is in your advantage. This is because the test2 oil company gains proportionally less than you do when their porosities are already better than yours. It also pays you to define a saturation/height function that gives more oil. Hence lowering the a value and making the b value more negative both improve your equity position.
    • 239 Solutions to Exercises Note that the prize for pushing for the right model could be an improvement in the value of your acreage of about 4% out of 17%, or a 23% improvement. The actual equity involved would change when the full mapping was performed, but it is likely that the improvement will remain roughly constant given that the two wells are indeed representative. Parameter Value Equity Value Equity Vshco Matrix density Fluid density A B 0.4 2.65 0.75 0.4 -0.25 0.199 0.191 0.194 0.211 0.206 0.6 2.67 0.95 0.5 -0.35 0.199 0.206 0.204 0.185 0.192 Base case: Equity GBV NPV HCPVarch HCPVcap test1 test2 0.394 0.283 0.167 0.199 0.606 0.717 0.833 0.801 CHAPTER 10: PRODUCTION GEOLOGY ISSUES Exercise 10.1: Dip Magnitude tan(a ) = (100 ) (25, 000 *.004) = 0.1 a = 5.7 degrees
    • 240 Well Logging and Formation Evaluation Exercise 10.2: Area Depth Graph The map should look something like this: 102 101.5 3120 3100 100.5 3220 3050 3025 100 3075 3090 3100 North (km) 101 3000 3200 99.5 3110 3100 3080 99 98 98.5 99 99.5 100 100.5 East (km) This leads to the approximate area depth relationship: Depth Area (m2) 3000 3025 3050 3075 3100 0 15,2305 484,606 896,907 1,66,1509 101 3000 101.5
    • 241 Solutions to Exercises Plotting this, together with the base of the sand, yields: Area (m2) 0 500,000 1,000,000 1,500,000 2,000,000 2980 3000 3020 GBV = 14.5* 106 m3 Depth (m) 3040 Top 3060 OWC 3070 Base 3080 3100 3120 3140 Taking the area above the OWC yields a GBV of 14.5*106 m3. NPV = GBV * porosity = 0.2 *14.5 *10 6 = 2.91*10 6 m 3 STOIIP = 2.91*10 6 * (0.8) 1.3 = 1.79 *10 6 m 3 ( = 11.26 MMstb) In order to take into account the change in saturation, the upper part of the sand should be treated separately from the lower half, and the two should be added together. This yields the following elements: GBV1 = 8.06 *10 6 m 3 ( 0 - 25 m above contact) GBV2 = 6.44 *10 6 m 3 ( 25 m + above contact) STOIIP = ( 0.2 1.3) * (8.06 *10 6 * 0.7 + 6.44 *10 6 * 0.9) = 1.76 *10 6 m 3 ( = 11.07 MMstb) This is lower than that seen when a single value of saturation is taken, because there is more rock volume lying close to the contact than in the crest of the structure. This illustrates why use of a saturation/height curve is important.
    • 242 Well Logging and Formation Evaluation CHAPTER 11: RESERVOIR ENGINEERING ISSUES Exercise 11.1: Density of Air In order to solve this we will use Equation 11.3, and calculate the mass and volume of 1000 moles of gas. The mass is simply the molecular weight, i.e., 29 kg. At standard conditions, Z = 1. The volume is given by: V = 1000 * 8.31* 288.5 (1.01325 *10 5 ) = 23.66 m 3 Hence the density = 29/23.66, or 1.226 kg/m3 Exercise 11.2: Material Balance of Undersaturated Oil Reservoir From equation 11.10 the composite compressibility is given by: C = ( 4.4 * 10 -5 * 0.2 + 10 * 10 -5 ) 0.8 = 1.36 * 10 -4 1 bar From Equation 11.11: N p = [10 7 * (1.3 - 1.25) + 10 7 * 1.3 * 1.36 * 10 -4 * (300 - 250)] 1.25 = 4.71 * 10 5 m 3 Exercise 11.3: Radial Flow Just apply Equation 11.19, making sure the units are correct. Hence: k = f * m * C k = 0.2 * (10 * 10 -3 ) * ( 2 * 10 -4 * 10 -5 ) ( 0.3 * 10 -12 ) = 13.3 t = r 2 * (1 4) * g * k * exp( 4 * p * k * h * DP ( Q * m)) = ( 0.15)2 * (1 4) * (1.781) * (13.3) * exp[( 4 * 3.14 * 0.3 * 10 -12 * 30) * (30 * 10 5 ) ( 200 ( 24 * 60 * 60) * 10 * 10 -3 )] = 306,934 secs (3.56 days)
    • 243 Solutions to Exercises Exercise 11.4: Horner Analysis Start by tabulating and plotting Pw vs ln[(t + Dt)/Dt]: Time (hr) 0 0.5 1 1.5 2 2.5 3 4 6 8 10 12 Pressure (bar) Ln[(24 + dt)/dt] Model [y = 226 -3*( x )] 205.32 217.89 221.95 223.05 223.53 223.81 223.96 224.2 224.48 224.68 224.82 224.96 0 1.39794 1.113943 0.954243 0.845098 0.763428 0.69897 0.60206 0.477121 0.39794 0.342423 0.30103 226 221.8062 222.6582 223.1373 223.4647 223.7097 223.9031 224.1938 224.5686 224.8062 224.9727 225.0969 227 226 225 223 222 221 220 219 218 217 1.5 0.5 1 In [(t+dt)/dt] 0 Pw (bar) 224 Buildup Model
    • 244 Well Logging and Formation Evaluation Fitting a straight line through the points corresponding to transient behaviour yields the model line, given by: Pw = 22 - 3.0 * ( ln[( 24 + T ) T ]) Hence the initial reservoir pressure is expected to be 226 bar. Using Equation 11.30, the product k*h can be estimated: L1 can be taken as 0, with P1 = 226 bar L2 can be taken at 1, with P2 = 223 bar Hence: k * h = (1 2) * Q * m * ( L1 - L2 ) [2 * p * ( P2 - P )] 1 = (1 2) * 200 ( 24 * 60 * 60) * 1.2 * 1 * 10 -3 * ( -1) [2 * 3.14 * -3 * 10 5 ] = 3.686 * 10 -13 m 2 Hence k = 36.86 mD. CHAPTER 12: HOMING-IN TECHNIQUES Exercise 12.1: Worked Field Example of Magnetostatic Homing In Start by entering the survey data from both wells in a spreadsheet. For the survey well, derive columns for the x, y and z components of r, the vector along the direction of the well bore. Convert these to unit vector components by dividing by r. Now derive the x, y and z components of unit vectors in the HS and HSR directions using equations 12.10 and 12.11. Derive the x, y and z components of the Earth’s field using equations 12.7–12.9. Convert these to the highside reference system using equations 12.12–12.14. Convert the raw tool readings to Bhs, Bhsr, and Bax. Note that since the tool face orientation is along the highside direction (there is no Ay component), Bx = Bhs, By = Bhsr and Bz = Bax. Derive Fxy, Fax, Ftot, HSdir and AXdir . You should end up with the following components:
    • 245 Solutions to Exercises Depth m md 2700 2710 2720 2730 2740 2750 2760 2765 2770 2772.5 2775 2777.5 2780 2782.5 2785 2787.5 2790 2792.5 2795 2797.5 2800 2805 2810 2820 Fxy mT Fax mT Ftot mT HSdir deg AXdir deg 0.62784 0.77294 0.42301 0.50055 0.5019 0.33035 0.54611 0.99398 2.83984 6.2726 11.1798 17.907 23.7569 23.9532 18.1464 11.919 8.68332 7.39378 5.40196 4.76053 3.47112 2.24769 1.71776 2.33087 -0.00412 0.03687 0.14493 -0.11071 -0.0754 -0.4646 -1.8051 -3.041 -5.5069 -6.72687 -7.94685 -6.11682 0.5832 6.64478 12.3364 11.4479 8.9695 4.7118 2.8941 1.3264 0.4787 0.11865 0.1186 -0.3621 0.62785 0.77382 0.44715 0.51265 0.50753 0.57007 1.8859 3.19932 6.19602 9.19762 13.7164 18.9229 23.7641 24.8577 21.9426 16.5263 12.4841 8.7675 6.12838 4.94186 3.50397 2.25082 1.72185 2.35883 349 307 311 140 164 341 274 271 243 245 241 243 241 241 233 213 188 169 165 164 159 146 133 39 90.4218 87.3133 71.1238 102.524 98.5936 144.659 163.25 161.982 152.798 137.071 125.47 108.915 88.6387 74.5334 55.8195 46.1783 44.0936 57.5212 61.8512 74.4686 82.1896 87.0224 86.094 98.8804 The field pattern is characteristic of a south monopole. Firstly it is noted that the maximum in Ftot occurs at 2781.5 ft in the survey well. This must be the point of closest approach between the survey well and pole. Using the formula for deriving the pole distance from the observed half-width of Ftot (equation 12.18), the distance to the pole is found to be 7.5 ft. Using the formula for converting HSdir into an inclination with respect to the horizontal (Equation 12.22) we find that f is 24.2 degrees. The true vertical depth of the pole is therefore: 2130.6 - 7.5 * sin( 24.2) = 2127.5 ft ss. Where 2130.6 is the true vertical depth at 2781.5 ft md in the relief well. The pole strength may be derived from the formula: Ftot = P ( 4 * p * x 2 )
    • 246 Well Logging and Formation Evaluation Using Ftot = 25.6 mT, x = 7.5 ft yields P = 1681 mWb. The field due to such a monopole may be modelled and compared to the measured readings. In order to do this, construct a spreadsheet as follows: For all the survey points model the field that would be seen in the highside reference system: Determine the x, y and z components of unit vectors in the HS, HSR, and RVEC directions. Calculate F.HSŸ, F.HSRŸ and F.RVECŸ, in order to determine Fhs, Fhsr and Fax. Calculate AXdir and HSdir from these quantities. Overlay the modelled and measured data. Initially the match is not very good. Because of the nature of the well trajectories (relief well passing to the right of the target well going north to south with an inclination of ~50 deg) the peaks can be aligned by moving the target well north or south. The height of the peaks may be adjusted by moving the target well east or west. By manually adjusting the assumed pole position on a trial-anderror basis it was found that the best fit occurred when the target well was moved 1.5 ft west and 8.5 ft north. After shifting the target well you should end up with the following components: Depth 2685.00 2697.00 2707.00 2717.00 2727.00 2740.00 2750.00 2760.00 2770.00 2780.00 2790.00 2800.00 2810.00 2820.00 2830.00 2840.00 2850.00 2860.00 TVD Fhs Fhsr Fax Ftot Fxy AXdir HSdir 2073.40 2080.70 2086.70 2092.70 2098.70 2105.70 2111.70 2117.60 2123.70 2129.70 2135.80 2141.90 2148.00 2154.10 2161.20 2167.30 2173.50 2179.70 -0.01 -0.01 -0.02 -0.03 -0.02 -0.08 -0.16 -0.52 -2.13 -12.92 -3.93 -0.76 -0.22 -0.02 -0.05 -0.03 -0.02 -0.01 -0.01 -0.02 -0.01 -0.03 -0.06 -0.11 -0.26 -0.75 -3.83 -20.63 -6.29 -1.19 -0.35 -0.15 -0.07 -0.05 -0.03 -0.02 -0.16 -0.20 -0.26 -0.35 -0.48 -0.80 -1.35 -2.67 -6.52 -4.66 8.35 3.32 1.60 0.92 0.58 0.40 0.30 0.23 0.16 0.20 0.26 0.35 0.49 0.81 1.39 2.82 7.85 24.78 11.17 3.61 1.65 0.93 0.58 0.41 0.30 0.23 0.01 0.02 0.02 0.04 0.06 0.14 0.30 0.91 4.38 24.34 7.42 1.41 0.42 0.15 0.09 0.06 0.03 0.02 175.14 174.77 175.29 173.16 172.93 170.54 167.40 161.22 146.16 100.88 41.63 22.95 14.63 9.38 8.99 7.91 6.10 5.33 227.71 234.06 220.24 230.48 254.70 234.72 238.53 235.38 240.89 237.97 238.02 237.43 238.16 262.97 234.87 237.10 237.94 237.96
    • Solutions to Exercises 247 The Field components should look as follows: 30 25 20 F (micro T) 15 Ftot Fxy Fax Ftot meas Fxy meas Fax meas 10 0 0 2660 2680 2700 2720 2740 2760 2780 2800 2820 2840 2860 2880 –5 –10 Depth It is seen that for AXdir and HSdir, good agreement is seen only in the vicinity of the pole. This is because the directions become poorly defined once the fields become weak far from the pole. The asymmetric behavior below 2781.5 ft may be attributed to a second, weaker pole along the target well axis. It may be seen that the addition of a second, north pole at 2147.5 ft in the target well, having a strength of 1000 mWb, could improve the match.
    • 248 Well Logging and Formation Evaluation Exercise 12.2: Interpretation of Electromagnetic Homing-In Data 400 350 300 Angle (deg) 250 AXDIR AXDIR meas HSDIR HSDIR meas 200 150 100 50 0 2650 2700 2750 2800 Depth ft md 2850 2900 From plotting the HSdir and H curve versus depth it is apparent that the point of closest approach occurs at 2795 ft md. Taking the width of the HSdir curve for -45 to +45 degrees yields D = 12 ft. Hence D = 12/2*tan(50) = 7.1 ft from equation 12.33. Taking the width of the Intensity curve at half the maximum, and subtracting a background signal of 52 mA/m/A yields D = 37 ft. This yields a distance of 5.0 ft from equation 12.34.
    • 249 Solutions to Exercises 350 360 310 300 260 Field Strength uA/m/A 250 210 200 Width at half max = 37 ft 160 Direction (deg) Width at half max = 12 ft Field strength Field direction 150 110 100 60 50 10 Closest approach at 2795 ft 0 2600 2650 2700 2750 2800 2850 –40 2900 Depth (ft) The difference cannot be explained, although it may be a combination of tool accuracy, noise, anisotropy, and variations in target current. Since most of these factors will affect the two AC magnetometers equally, probably the range determined from analyzing HSdir is the more accurate of the two. CHAPTER 13: WELL DEVIATION, SURVEYING, AND GEOSTEERING Exercise 13.1: Formation Dip from Up/Down Logs Using Equation 13.1 we see that the relative dip is: q = arctan[(8.5 12 ) * (1 3.281) 2] = 6.2 degrees
    • 250 Well Logging and Formation Evaluation We are in a Case D situation. The apparent formation dip is given by: a = 6.2 + 90 - 95 = 1.2 degrees from Equation 13.5 Using Equation 13.6 with g = 40 degrees we find: tan( D ) = tan (1.2 ) cos( 40 ) = 0.027 Hence D = 1.57 degrees.
    • A P P E N D I X 4 ADDITIONAL MATHEMATICS THEORY For readers who do not have a mathematics, engineering, or physics degree, some of the basic mathematical principles assumed in this book may be problematic. Therefore, this Appendix is designed to provide a fuller explanation of some of the theoretical derivations used in the chapters. A4.1 CALCULUS Differentiation is the taking of the gradient of a function with respect to one of the input variables. Start by considering the function: y = a * x + b. This is the equation of a straight line having a gradient of a and intercept on the y axis at b. The differential of y with respect to x is a function that describes the rate of change of y with x. It is denoted by dy/dx, where d represents the infinitesimally small increments of y and x. For the function given: dy dx = a. For most functions that engineers encounter, the differentials are simply known by heart, or can be looked up in mathematical handbooks. Table A4.1 gives most of the functions one is likely to come across: It is also possible to take the differential of dy/dx, in which case the result is referred to as d2y/dx2 or d/dx(dy/dx). Where a function depends 251
    • 252 Well Logging and Formation Evaluation y a 1 b x Figure A4.1 Equation of a Straight Line Table A4.1 Function y = xn + a (n π 0) y = ex y = log(x) (log is natural logarithm base e) y = sin(x) (x must be in radians = deg*p/180) y = cos(x) (x must be in radians = deg*p/180) y = tan(x) y = ax dy/dx n*xn-1 ex 1/x cos(x) -sin(x) sec2(x) (sec = 1/cos) ax*log(a) on more than one input variable, the situation is a bit more complex. Consider the function: t = a* x + b* y In order to derive dt/dx you also need to know how y will vary with x, if at all. In most engineering applications, x and y might be parameters such as pressure or temperature, which one can control in a laboratory. A special notation convention is used when the differential with respect to one variable is derived while keeping the other variables constant. Hence the partial differential of t with respect to x while keeping y constant is denoted as ∂t/∂x, or sometimes ∂t/∂x|y.
    • 253 Additional Mathematics Theory For the function given ∂t/∂x = a, the constant terms becoming zero on differentiation. Integration is just the opposite of differentiation. While taking the differential of a function of one variable yields the gradient of a graph of y vs. x, integrating the function yields the area under the graph (from the curve to the y = 0 axis). Consider again the function y = a*x + b. The integral of y with respect to x is denoted by: Ú y dx = Ú (a * x + b) dx = 0.5 * a * x 2 + bx + c where c is a constant and the ∫ is like a drawn-out S, indicating that the summation is made over infinitessimally small increments of dx. Since integration is the opposite of differentiation: Ú ( dy dx) dx = y + c. The constant c arises because the gradient (dy/dx) contains no information about any fixed offset of y from the y = 0 axis (which disappears during differentiation). In order to determine the area under a graph of y vs. x, one needs to specify a start and end point for x. These are placed at the bottom and top of the ∫ sign. The integral becomes a definite integral. As with differentiation, most engineers have committed to memory the common indefinite integrals they are likely to need. Table A4.2 shows those commonly used. For a definite integral, the normal procedure is to first evaluate the indefinite integral, then subtract the value of the function at the start value from that at the end value to get the area under the graph. Hence, for our Table A4.2 Function y = xn + a (n π -1) y = ex y = log(x) (log is natural logarithm base e) y = 1/x y = sin(x) (x must be in radians = deg*p/180) y = cos(x) (x must be in radians = deg*p/180) y = tan(x) y = ax ∫ ydx (1/n + 1)*xn+1 + a*x + c ex + c x*(log(x) - 1) + c log(x) + c -cos(x) + c sin(x) + c -log(cos(x)) + c ax/log(a) + c
    • 254 Well Logging and Formation Evaluation example function y = a*x + b evaluated between x1 and x2, the integral becomes: Ú x2 x1 2 2 ( a * x + b)dx = [0.5 * a * x 2 + bx + c] = 0.5 * a * ( x2 - x1 ) + b * ( x2 - x1 ) In many real engineering problems, data are presented as sampled at discrete intervals (e.g., 0.5-ft sampling increment for logs) and cannot be described by simple mathematical functions. For these data, a numerical differentiation or integration may also be performed without resort to calculus. Say, for instance, one wanted to make a differential of a GR log with respect to depth. The procedure would be to simply take the difference between successive data values at each increment and divide by the depth increment. Taking the integral would involve just adding successive data values multiplied by the depth increment. A4.2 SPECTRAL (FOURIER) ANALYSIS For any wireline log sampled in depth, it is possible to think of the log as being composed of a complex mixture of cosine waves that, when added together in the right proportions, yield the log. The cosine functions will have the form: yi = Ai * cos(2 * p * x l i + fi ) where Ai = the amplitude of the component i (1/li), or ki, = the wavenumber of the component i fi = the phase of the component i. If L(x) is the complete log, we can say: L( x ) = Syi. Spectral analysis is the mathematical determination of the set of Ai and fi as a function of ki. The determination of the spectra is performed using computer algorithms, which will not be discussed here. The range of ki that needs to be used is 0 (corresponding to a cosine wave of infinite wavelength) to 1/sample increment (since variations at a smaller scale than the sampling increment cannot be detected anyway).
    • 255 Additional Mathematics Theory 120 100 GR 80 60 40 20 0 0 20 100 150 200 250 300 200 250 300 200 250 300 A (I) Depth (m) 500 450 400 350 300 250 200 150 100 50 0 0 20 100 150 phase Wave number 2 1.5 1 0.5 0 –0.5 0 –1 –1.5 –2 20 100 150 Wave number Figure A4.2 Example of Spectral Analysis Hence a single log may be broken down into an amplitude and phase spectrum, which together give you the relative proportions and phase of each cosine wave as a function of wavenumber needed to add up to the log. This is illustrated in Figure A4.2. In the event that the log data are sampled in time rather than depth, the spectra are a function of frequency rather than wavenumber. If the log were in fact a sound wave, such as a piece of music, the amplitude spec-
    • 256 Well Logging and Formation Evaluation trum would correspond to the frequency content of the music. Taking out some components in the frequency spectrum is analagous to using a graphic equalizer on a household stereo. Filtering usually refers to manipulating the amplitude or phase spectra to take out unwanted components. Since the transform process is reversible, a new “log” may be constructed from the filtered components. A4.3 NORMAL (GAUSSIAN) DISTRIBUTIONS The normal probability curve is defined by: p( x ) = [exp( -0.5 * ( x - m )2 s 2 )] ( (2 * p ) * s ) (A4.1) The mean of the distribution is given by m and the variance by s. This function is shown in Figure A4.3, for a mean of zero and variance of 1. The probability that a value lies within the range between x1 and x2 is given by: p = (2 (2 * p ) ) * Ú x2 x1 exp( -0.5 * ( x - m )2 s 2 )dx (A4.2) It so happens that many distributions occurring in nature are normal, so it is often useful to just calculate the mean and variation of a given distribution and thereafter assume that the probabilities of new values occurring within a given range can be calculated using equation A4.2. 0.45 0.4 0.35 p(x) 0.3 0.25 0.2 0.15 0.1 0.05 0 –4 –2 0 x 2 Figure A4.3 The Normal Distribution 4
    • Additional Mathematics Theory 257 In particular, it is worth noting that 68.3% of values will fall within 1 standard deviation (SD is the square root of the variance), 95.4% within 2 SD and 99.7% within 3 SD of the mean. A4.4 VECTOR MECHANICS An understanding of the basics of vector mechanics is essential to being able to deal with components in the highside refence system, as discussed in Chapter 12. Consider a cartesian reference system defined by three orthogonal axes: x, y, and z. A vector is essentially just a way of writing directions, in terms of how far you have to travel in the x, y, and z directions to get from one point to another in space. Hence the vector linking the origin to the point A located at (a1, a2 , a3) is denoted by a, and has the components: Ê a1 ˆ Á a2˜ Á ˜ Ë a3¯ 2 2 The length of the vector, denoted by a, is given by (a12 + a2 + a3 ) . A unit vector is one that has a length of 1. To convert a to a unit vector, each 2 2 component would have to be divided by (a12 + a2 + a3 ) and the vector would then be designated by â. Taking the scalar product (sometimes called the dot product) of two vectors allows the angle between them to be determined: a . c = a * c * cos(q ) = (a1 * c1 + a2 * c2 + a3 * c3 ) (A4.3) where a, c are the magnitude of a and c, and q is the angle between the vectors. The vector product of two vectors (sometimes called the cross product) generates a new vector that is orthogonal (i.e., at right angles) to both. Hence: Ê a1 ˆ Ê c1 ˆ Ê a2 * c3 - a3 * c2ˆ Á a2˜ Ÿ Á c2˜ = Á a3 * c1 - a1 * c3 ˜ Á ˜ Á ˜ Á ˜ Ë a3¯ Ë c3¯ Ë a1 * c2 - a2 * c1 ¯ (A4.4) The vector defined by a Ÿ c has magnitude a*c* sin (q). The direction of the vector product will follow a right-hand corkscrew rule as one goes
    • 258 Well Logging and Formation Evaluation from a to c. Hence, say a lies in a northerly direction and c in an easterly direction; the cross-product would point downward into the Earth. Of particular reference to surveying is the fact that taking the vector product of a unit gravity vector with a vector in the hole direction yields a horizontal vector 90 degrees to the right of highside in the sensor plane. Taking the vector product of this vector with a vector in the hole direction yields a vector in the direction of highside. A4.5 PROBABILITY THEORY It may be helpful for readers who are involved with VOI calculations to have more background information on probability theory to understand better the concepts of EMV and reliability. Some of these will be explained in this section. Suppose that you have discovered an oil field. This field has many uncertainties surrounding it. However, you have determined a field development plan that you intend to carry out, and wish to know the EMV of such a plan. In reality, depending on the actual true nature of the field, your plan may be either a very good one, a very bad one, or somewhere in between. While the true nature of the field may obviously have an infinite number of different states, consider for now that there are N possible states that more or less encompass all the range of possibilities. For a particular state i (out of the N possibilities), your field development plan will yield a value of NPV(i), this being the present value of the net revenue minus expenditure over the life of the field. The state i has a probability P(i) of being close to the true state of the field. Clearly it must be true that  N i =1 P(i ) = 1.0 (A4.5) The EMV will be given by: N EMV = Âi =1 (i ) * NPV(i ) (A4.6) Some of the NPV(i) may be negative (e.g., if the field is much smaller than originally thought), and some may be very positive (if the field is larger than expected). The final EMV should certainly be positive, or else the whole development would not be worth embarking on. Now consider that someone proposes an amendment to the field development plan. An example might be the addition of a data acquisition
    • Additional Mathematics Theory 259 program such as thermal decay logging. This program will not, of course, change P(i), but it will change NPV(i). More money will be spent (reducing NPV(i)) but in return, if the data are reliable and useful (which they may be for only some of all possible field states), there will result an increase in revenue or decrease in other costs, having the net effect of increasing NPV(i). Since the NPV(i) has changed, the EMV will change, to EMV¢. The EMV of the proposed change, which we will call DEMV is given by: DEMV = EMV ¢ - EMV (A4.7) It is important to note that DEMV, irrespective of any issues concerning reliability, depends on all the possible states of the field, not just the base case. In order to introduce the concept of reliability, it will be much simpler to consider for now that the field has only two possible states, which will be denoted as S1 and S2. The EMV of the field is then approximated by: EMV = P( S1 ) * NPV( S1 ) + P( S2 ) * NPV( S2 ). (A4.8) Now consider a proposal for a change to the FDP. This will involve the acquisition of data at a cost of Z and be such that a parameter C will be determined as being true or false. C is such that: 1. If C is true, the field is known to be definitely in state 1. If C is false, the field is known to be definitely in state 2. Knowing which state the field is in would allow the FDP to be optimized. 2. If C is true, the FDP may be optimized to yield a new NPV given by NPV(S1 and C). the field being in state S1. 3. Likewise if C is false, the FDP may be optimized to yield a new NPV given by NPV(S2 and C¢), the field being in state 2. The change in the EMV is given by: DEMV = - Z + P( S1 ) * NPV( S1 and C ) + P( S2 ) * NPV( S2 and C ¢ ) - EMV. (A4.9) For the change to be worthwhile, we clearly require that at least one of the NPV(S1 and C) or NPV(S2 and C¢) be greater than NPV(S1) or NPV(S2).
    • 260 Well Logging and Formation Evaluation The VOI is clearly given by VOI = (DEMV - Z). Now consider the effect of reliability. This means in effect that sometimes the parameter C will be found to be true even though the field is in state 2, and vice versa. The reliability is expressed via: R = P(C S1 ) i.e., the probability that C is found to be true when the field is indeed in state 1. For simplicity we will also assume that this is the same as P(C¢/S2), i.e., the probability that C is found to be false when the field is indeed in state 2. The introduction of R clearly leads us to have to consider the additional NPV scenarios: 1. NPV(S1 and C¢): the NPV realized when the field is thought to be in state 2 but is actually in state 1. 2. NPV(S2 and C): the NPV realized when the field is thought to be in state 1 but is actually in state 2. We will now calculate DEMV, introducing these additional scenarios: DEMV = - Z + P( S1 ) * [P( C S1 ) * NPV( S1 and C) + P( C¢ S1 ) * NPV( S1 and C¢)] + P( S2 ) * [P( C¢ S2 ) * NPV( S2 and C¢) + P( C S2 ) * NPV( S2 and C)] - EMV (A4.10) Using our definition of R: DEMV = - Z + P( S1 ) * [ R * NPV( S1 and C) + (1 - R) * NPV( S1 and C¢)] + P( S2 ) * [ R * NPV( S2 and C¢) + (1 - R) * NPV( S2 and C)] - EMV (A4.11) Another way to look at this is that we have four possible combinations of C, C¢, S1, and S2, each having a probability and NPV associated with it. The EMV is therefore: DEMV = P( S1 and C) * NPV( S1 and C) + P( S2 and C) * NPV( S2 and C) + P( S1 and C¢) * NPC( S1 and C¢) + P( S2 and C¢) * NPV( S2 and C¢) - Z - EMV (A4.12)
    • Additional Mathematics Theory 261 Also P( S1 and C) + P( S2 and C¢) + P( S1 and C¢) + P( S2 and C¢) = 1 (A4.13) Now P(C and S1), the probability of both C and S1 occurring, is given by: P( C and S1 ) + P( C S1 ) * P( S1 ) = R * P( S1 ) and so on for the other combinations. Replacing P(C and S1) and similar terms in equation A4.12 yields back the same result as equation A4.7. It is often useful to make a plot of DEMV as a function of R. In this way it is possible to determine the value of R for which a particular data acquisition campaign becomes viable. The above concepts can obviously be extended to cover more than two states, and specialized software is available that will allow the EMV to be calculated relatively simply. Note that the reliability of the tool has been defined as P(C/S1) etc. One is also interested to know P(S1/C), i.e., the probability of the field being in state 1 given that the tool yields a result C. To make this conversion it is necessary to use Bayes’ theorem. This uses the fact that: P( C and S1 ) = P( C S1 ) * P( S1 ) = P( S1 and C) = P( S1 C) * P( C) (A4.14) P( C) = P( C S1 ) * P( S1 ) + P( C S2 ) * P( S2 ). (A4.15) Combining these equations: P( S1 C) = [P( S1 ) * P( C S1 )] [P( S1 ) * P( C S1 ) + P( S2 ) * P( C S2 )] (A4.16) P( S1 ) * R [P( S1 ) * R + P( S2 ) * (1 - R)], since R = P( C S1 ) (A4.17) Likewise: P( S1 C¢) = [P( S1 ) * P( C¢ S1 )] [P( S1 ) * P( C¢ S1 ) + P( S2 ) * P( C¢ S2 )] (A4.18) P( S1 ) * (1 - R) [P( S1 ) * (1 - R) + P( S2 ) * R] (A4.19) P( S2 C) = [P( S2 ) * P( C S2 )] [P( S2 ) * P( C S2 ) + P( S1 ) * P( C S1 )] (A4.20) P( S1 ) * (1 - R) [P( S1 ) * (1 - R) + P( S2 ) * R] (A4.21)
    • 262 Well Logging and Formation Evaluation P( S2 C¢) = [P( S2 ) * P( C¢ S2 )] [P( S2 ) * P( C¢ S2 ) + P( S1 ) * P( C¢ S1 )] (A4.22) P( S2 ) * R [P( S2 ) * R + P( S1 ) * (1 - R)], since R = P( C¢ S2 ). (A4.23) Note that it is always true that: P( S1 C) + P( S2 C) = 1 and P( S1 C¢) + P( S2 C¢) = 1. Note also that only in the special case that: P( S1 ) = P( S2 ) = 0.5 is it true that: P( S1 C) = P( C S1 ) A4.6 LEAST SQUARES FIT AND CORRELATION Consider a series of points (x1, y2) . . . (xn, yn). When plotted these points may lie approximately on a straight line or a curve or be scattered randomly. Least squares fit is a way to find the coefficients of a function approximating the behavior of the data. Consider data which may be approximated by the formula y = a*x + b. The line yielded by this equation is known as the line of regression of y on x. Forming the sum of the squares of all the deviations of the given numerical value of y from the theoretical values: S = S( y - a * x - b ) 2 (A4.24) S is minimized when ∂S/∂a = ∂S/∂b = 0. This occurs when: S 2 * x * ( y - a * x - b ) = 0 and S 2 * y * ( y - a * x - b ) = 0. (A4.25) Solving these equations: a = [n * Sx * y - Sx * Sy] [n * Sx 2 - ( Sx )2 ] (A4.26) b = [ Sy * x 2 - Sx * Sx * y] [n * Sx 2 - ( Sx )2 ] (A4.27) where n is the number of samples.
    • Additional Mathematics Theory 263 The regression of y on x assumes that the x values in the data are always correct and that the scatter occurs in the y variable. Similarly, the line of regression of x on y may be derived simply by first setting x = (1/a)*y + (-b/a) and using equations 13.24–13.27 in an identical way. A set of points on a plane may exhibit only a trend rather than a close approximation to a straight line. The extent to which the points are linearly related is specified quantitatively by the correlation coefficient. This is given by: r = a * sx s y (A4.28) where sx, sy are the variances of the x and y values about their mean. In the more general case where any function is used to describe y on x: r = s xy (s x s y ) (A4.29) where sxy is the covariance of x and y given by: s xy = S( x - mx ) * ( y - my ) n (A4.30) and mx, my are the means of the x and y values. The correlation coefficient will be 1 when the match is perfect between the model and the data, and zero if there is no correlation. In practice, it is usually easiest to do correlation within an ExcelTM spreadsheet. A convenient way to do the fitting, where multiple variables and a nonlinear equation is being used is as follows: Set trial values of the relevant coefficients in cells in the spreadsheet. Using these coefficients, calculate the model result (y¢) at all values of x for which a y value is available for comparison. In a new column calculate (y - y¢)2 for each data point At the bottom of this column create the sum of all the values. The fit will obviously be optimized when the set of coefficients is found that minimizes this sum. This set can be found automatically within ExcelTM using the Goal SeekTM function. Depending on the complexity of the equation and number of variables, it may be necessary to constrain the ranges of the coefficients. ExcelTM can also return the correlation coefficient.
    • A P P E N D I X 5 ABBREVIATIONS AND ACRONYMS AHD AI API AVO B/D Bg BHA BHT BOE BOP bopd BQv BS BU BV BVI CAL CBL CCL CEC CHP COI CPI DD DF Alonghole (sometimes used in place of measured depth) Acoustic impedance American Petroleum Institute (also units used for gamma ray) Amplitude versus offset (of seismic traces) Barrels per day Gas volumetric factor (in scf under standard conditions per scf in reservoir) Bottomhole [drilling] assembly Bottomhole temperature Barrel of oil equivalent Blowout Preventer Barrels of oil per day The product of B, the equivalent counter-ion conductance, and Qv, the cation exchange capacity per unit pore volume. Bit size Buildup Bulk volume Bulk volume of irreducible water Caliper Cement bond log Casing collar locator Cation exchange capacity Casing head pressure Cost of information Computer processed interpretation Driller’s depth Derrick floor 264
    • Abbreviations and Acronyms DHI DOP DT DTS Ec EHC EI Ek EOR ESP FBU FFI FOL FPI FRF FSI FWL GBV GC GDT GIIP GL GOC GOR GR GUT GWC HCPV HDT HI HIIP HUD HWC ID JV k KB KCl 265 Direct hydrocarbon indicator Dropoff point Delta time (inverse of compressional velocity as measured by sonic tool) Delta time shear Eckert number Equivalent hydraulic conductivity Elastic impedance Kinetic energy Enhanced oil recovery Electric submersible pump Formation buildup Formation fluid index Free oil level Free point indicator Formation resistivity factor Formation strength indicator Free water level Gross bulk volume Gas chromatography Gas down to Gas initially in place Ground level Gas/oil contact Gas/oil ratio Gamma ray Gas up to Gas/water contact Hydrocarbon pore volume Higher dipmeter tool Hydrogen index Hydrocarbons initially in place Holdup depth Hydrocarbon/water contact Inner diameter Joint venture Permeability Kelly bushing Potassium chloride
    • 266 Well Logging and Formation Evaluation kh Km KOP kv Horizontal permeability Matrix modulus Kickoff point Vertical permeability LCM LST LWD Lost circulation material Limestone Logging while drilling MD MWD Measured depth Measurement while drilling NMR NRV NTG Nuclear magnetic resonance Net rock volume Net to gross OBM OD ODT OUT OWC Oil-based mud Outer diameter Oil down to Oil up to Oil/water contact Pc Pe PE Pf PHIT PI psia psig PT PU PVT Q QA QC Qv RB rb/stb rhog Rm Rmf ROP Capillary pressure Photoelectric effect (tool) Petroleum engineering Formation pressure Matrix-corrected total porosity index Productivity index Pounds per square inch absolute Pounds per square inch per gauge Production technology Porosity units Pressure, volume, temperature Flow rate Quality assurance Quality control Cation exchange capacity per unit pore volume Relative bearing Reservoir barrels/stock tank barrels Grain density Mud resistivity Mud filtrate resistivity Rate of penetration
    • Abbreviations and Acronyms ROS RQI Rt RU Rw SCAL SEM SG SH SO SOR SP SPE SST STOIIP Sw TC TD TDA Te TG TOC TVD TVDss Tw TWT TZ UTC UV VOI Vp Vs Vsh VSP WC WL WOB WST WUT Residual oil saturation Reservoir quality index True resistivity Rig up Water resistivity Special core analysis Scanning electron miscroscopy Gas saturation Hydrocarbon saturation Oil saturation Residual oil saturation Spontaneous potential Society of Petroleum Engineers Sandstone Stock tank oil initially in place Water saturation Total carbon Total depth Time domain analysis Echo spacing time for NMR logging Total gas Top of cement True vertical depth True vertical depth subsea Polarization wait time for NMR logging Two-way time Time vs. depth Ultimate technical cost Ultraviolet Value of information Compressional velocity Shear velocity Shale volume Vertical seismic profile Water cut Wireline Weight on bit Well shoot test Water up to 267
    • A P P E N D I X 6 USEFUL CONVERSION UNITS AND CONSTANTS Depth 1 m = 3.281 ft Volume 1 barrel = 0.15899 cubic meters = 5.614 cubic feet = 42 US gallons 1 US gallon = 0.1337 cu ft Pressure/Density 1 psi = 0.06895 bar = 0.068065 atm = 6895 N/m2 (Pascals) 1 psi/ft = 2.3095 g/cc = 22.6 kPa/m 1 g/cc = 8.35 lb/gal Temperature Conversion from °C to °F: °C = (°F - 32)*5/9 °C = °K + 273.16 °Rankine = 1.8*°K = °F + 460 Permeability 1 darcy = 10-12 m2 268
    • GR (API) Low Low High High High High Low Low Low 0 0 0 Lithology Sandstone matrix Limestone matrix Chlorite Illite Kaolinite Montmorillonite Dolomite Anhydrite Salt Brine Oil Gas 2.65 2.71 2.79 2.52 2.41 2.12 2.88 2.98 2.03 1–1.1 0.60–1.0 0.1–0.5 Rhob (g/cc) AC (ms/ft) 51–56 44–48 50–150 50–150 50–150 50–150 39–44 45–55 180–190 189 210–240 500–1500 CNL (NPU) -5 to -3 0 30–40 30–40 30–40 30–40 2–6 0 100 100 70–100 10–50 Table 6.1 High High Low Low Low Low High High High 0–• Very high Very high Resistivity (ohms) 1.8 5.1 6.3 1–5 1–5 1–5 3.1–3.2 5–5.1 4.65 0.36 0.12–0.13 Low Pe 4.79 13.77 17.58 8.69 4.41 4.32 9.00 14.95 U (barn/cc) 0.96 0.11–0.12 Low 2.65 2.71 2.79 2.52 2.41 2.12 2.86 2.96 re g/cc 1.185 0.95–0.97 0.1–0.5 Useful Conversion Units and Constants 269
    • 270 Well Logging and Formation Evaluation Viscosity 1 centipoise (cp) = 10-3 Pa.s Concentration To convert ppm of [Cl –] to [NaCl], you must multiply by: molecular weight of NaCl molecular weight of Cl = (23 + 17) 17 = 2.35. Other useful molecular weights are: K (potassium): 39 Ca (calcium): 40 Mg (magnesium): 24 Br (bromium): 80 Note that 1 g/l = 1000 ppm Conversion from Bottomhole to Surface Conditions Oil: Bo = 1.2–1.6 reservoir barrels/stock tank barrels Gas: Bg = 0.8–1.2 reservoir barrel/scf Resistivity of Saline Solutions as a Function of Temperature Rw is approximated by: Rw 2 = Rw1 * (T1 + k ) (T2 + k ) where k = 6.77 when T is in °F and k = 31.5 when T is in °C. Properties of Some Common Lithologies Note that U = Pe*re, where Pe is the photoelectric absorption index as displayed on a typical log and U is the volumetric photoelectric absortion index. U is typically used in forward modeling of multimineral models, since it is independent of porosity.
    • A P P E N D I X 7 CONTRACTOR TOOL MNEMONICS Schlumberger Tool Mnemonics Mnemonic CALAA CALB CALC CALD CALF CALFA CALGA CALJ CALJB CALM CALN CALQ CALQA CALQB CALQC CALQT CALR CALS CALT CALU CALV CALW CALY CALYA CALZ CBT CBTE CCL CCLAF CCLAG CCLAJ Type CCL CCL CCL CCL CCL CCL CCL CCL CCL CCL CCL CCL CCL CCL CCL CCL CCL CCL CCL CCL CCL CCL CCL CCL CCL CBT CBT CCL CCL CCL CCL Mode WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Application Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Cement_Evaluation Cement_Evaluation Collar_Locator Collar_Locator Collar_Locator Collar_Locator 271 Description Casing Anomaly Locator—AA Casing Anomaly Locator—B Casing Anomaly Locator—C Casing Anomaly Locator—D Casing Anomaly Locator—F Casing Anomaly Locator—FA Casing Anomaly Locator—GA Casing Anomaly Locator—J Casing Anomaly Locator—JB Casing Anomaly Locator—M Casing Anomaly Locator—N Casing Anomaly Locator—Q Casing Anomaly Locator—QA Casing Anomaly Locator—QB Casing Anomaly Locator—QC Casing Anomaly Locator—QT Casing Anomaly Locator—R Casing Anomaly Locator—S Casing Anomaly Locator—T Casing Anomaly Locator—U Casing Anomaly Locator—V Casing Anomaly Locator—W Casing Anomaly Locator—Y Casing Anomaly Locator—YA Casing Anomaly Locator—Z Cement Bond Tool Cement Bond Tool—E Casing Collar Locator Casing Collar Locator—AF Casing Collar Locator—AG Casing Collar Locator—AJ
    • 272 Mnemonic CCLAK CCLAL CCLAM CCLAN CCLAP CCLAR CCLL CCLLB CCLN CCLNB CCLX CET CETB CETC CETD CETE CETF CETG CETH CETJ CIT CITA CPET DCALA ETT ETTD ETTDB FTGT FTGTB GFAB GFAC GPT GPTA GUN GUN1 GUN2 GUN3 GUN4 GUN5 GUN6 GUN7 GUN8 GUN9 HCMT HCMT-A MFCT MPBC MPBCAA MPBT MPD Well Logging and Formation Evaluation Type CCL CCL CCL CCL CCL CCL CCL CCL CCL CCL CCL CET CET CET CET CET CET CET CET CET CIT CIT CPET CCL ETT ETT ETT FTGT FTGT GFA GFA GPT GPT GUN GUN GUN GUN GUN GUN GUN GUN GUN GUN HCMT HCMT MFCT MPBC MPBC MPBT MPD Mode WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Application Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Collar_Locator Cement_Evaluation Cement_Evaluation Cement_Evaluation Cement_Evaluation Cement_Evaluation Cement_Evaluation Cement_Evaluation Cement_Evaluation Cement_Evaluation Casing_Inspection Casing_Inspection Casing_Inspection Collar_Locator Casing_Inspection Casing_Inspection Casing_Inspection Casing_Inspection Casing_Inspection Nuclear Nuclear Nuclear Nuclear Perforating Perforating Perforating Perforating Perforating Perforating Perforating Perforating Perforating Perforating Cement_Evaluation Cement_Evaluation Casing_Inspection Mechanical Mechanical Mechanical Auxiliary Description Casing Collar Locator—AK Casing Collar Locator—AL Casing Collar Locator—AM Casing Collar Locator—AN Casing Collar Locator—AP Casing Collar Locator—AR Casing Collar Locator Casing Collar Locator—LB Casing Collar Locator Casing Collar Locator Casing Collar Locator Cement Evaluation Tool Cement Evaluation Tool Cement Evaluation Tool Cement Evaluation Tool Cement Evaluation Tool Cement Evaluation Tool Cement Evaluation Tool Cement Evaluation Tool Cement Evaluation Tool Casing Inspection Tool Casing Inspection Tool Corrosion Protection Evaluation Tool Digital Casing Collar Locator—A Electromagnetic Thickness Tool Electromagnetic Thickness Tool Electromagnetic Thickness Tool Tubing Geometry Tool Tubing Geometry Tool—B Surface Powered Gamma Ray (W5) Surface Powered Gamma Ray (W7) Gamma Ray Perforating Tool Gamma Ray Perforating Tool—A GUN Perforating Gun Perforating Gun Perforating Gun Perforating Gun Perforating Gun Perforating Gun Perforating Gun Perforating Gun Perforating Gun HPHT Slim Cement Mapping Tool HPHT Slim Cement Mapping Tool (A) Multi-Finger Caliper Tool Mechanical Plugback Cartridge Mechanical Plugback Cartridge—AA Mechanical Plug Back Tool Magnetic Positioning Device
    • 273 Contractor Tool Mnemonics Mnemonic MPDH MPSU MPSUAA MPSUBA MPSUBB MPSUCA MPSUCB MWPS MWPT NDT NDTA NDTB PAT PAT-G PATA PATB PATC PATD PATE PATG PERFO PGGA PGGB PGGC PGGCC PGGD PGGT PHAT PLUG SAFE SCALA SCCL SCCL-A SCMT SCMT-A SPC SPCA SPCB SPGC SPPT SPPTA UCI USIT Type MPD MPSU MPSU MPSU MPSU MPSU MPSU MWPT MWPT NDT NDT NDT PAT PAT PAT PAT PAT PAT PAT PAT PERFO PGGT PGGT PGGT PGGT PGGT PGGT PHAT PLUG SAFE CCL SCCL SCCL SCMT SCMT SPC SPC SPC SPC SPPT SPPT UCI USIT Mode Application Description WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Auxiliary Mechanical Mechanical Mechanical Mechanical Mechanical Mechanical Perforating Perforating Nuclear Nuclear Nuclear Casing_Inspection Casing_Inspection Casing_Inspection Casing_Inspection Casing_Inspection Casing_Inspection Casing_Inspection Casing_Inspection Perforating Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Casing_Inspection Mechanical Perforating Collar_Locator Collar_Locator Collar_Locator Cement_Evaluation Cement_Evaluation Perforating Perforating Perforating Perforating Mechanical Mechanical Scanning Scanning Magnetic Positioning Device Mechanical Plugback Setting Unit Mechanical Plugback Setting Unit—AA Mechanical Plugback Setting Unit—BA Mechanical Plugback Setting Unit—BB Mechanical Plugback Setting Unit—CA Mechanical Plugback Setting Unit—CB Measurement While Perforating Sonde Measurement While Perforating Tool Neutron Depth Tool Neutron Depth Tool—A Neutron Depth Tool—B Pipe Analysis Tool Pipe Analysis Tool—GA Pipe Analysis Tool—A Pipe Analysis Tool—B Pipe Analysis Tool—C Pipe Analysis Tool—D Pipe Analysis Tool—E Pipe Analysis Tool—G Perforating (Dummy) Tool Powered Gun Gamma Ray—A Powered Gun Gamma Ray—B Powered Gun Gamma Ray—C Powered Gun Gamma Ray—CC Powered Gun Gamma Ray—D Powered Gun Gamma Ray Pit and Hole Analysis Tool PLUG Slapper-Activated Firing Equipment Casing Collar Locator Slim Casing Collar Locator Slim Casing Collar Locator—A Slim Cement Mapping Tool Slim Cement Mapping Tool—A Selective Perforating Cartridge Selective Perforating Cartridge—A Selective Perforating Cartridge—B Selective Perforating Cartridge Production Packer Tool Production Packer Tool Ultrasonic Corrosion Imager Ultrasonic Imaging Tool Back-off Tool Correlatable Electromagnetic Recovery Tool, 1–11/16 Inch Correlatable Electromagnetic Recovery Tool, 3–3/8 Inch Drilling/Back-off BO CERB BO CERT WIRELINE WIRELINE Special_Purpose Special_Purpose CERC CERT WIRELINE Special_Purpose
    • 274 Mnemonic Well Logging and Formation Evaluation Type Mode Application CERD CERT WIRELINE Special_Purpose CERE CERT WIRELINE Special_Purpose CERT CERT WIRELINE Special_Purpose CERTA CERT WIRELINE Special_Purpose CERTB CERT WIRELINE Special_Purpose CERTC CERT WIRELINE Special_Purpose FPIT FPITA FPITC FPIT FPIT FPIT WIRELINE WIRELINE WIRELINE Special_Purpose Special_Purpose Special_Purpose FPITD FPITE FPIT FPIT WIRELINE WIRELINE Special_Purpose Special_Purpose GSTA GSTA-CBB IAB IAB4-AA IAB4I-AA IAB4I-AB IAB6I-AA IAB8I-AA IAB9I-AA IDEAL_SURF GSTA GSTA IAB IAB IAB IAB IAB IAB IAB IDEAL_SURF MWD MWD MWD MWD MWD MWD MWD MWD MWD MWD Combination Combination Geometry Geometry Geometry Geometry Geometry Geometry Geometry Auxiliary M10 MWD SHARP SLIM1 VBHA-AA MWD MWD SHARP SLIM1 VIPER MWD MWD MWD MWD MWD MWD MWD MWD MWD MWD VIPER VIPER MWD MWD AMS AST BGIC BGS BGT BGT BGT BHTV BHTV BHTV WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Auxiliary Scanning Geometry Geometry Geometry Geometry Geometry Scanning Scanning Scanning Description Correlatable Electromagnetic Recovery Tool, 2–1/8 Inch Correlatable Electromagnetic Recovery Tool, 2–3/4 Inch Correlatable Electromagnetic Recovery Tool Correlatable Electromagnetic Recovery Tool—A Correlatable Electromagnetic Recovery Tool—B Correlatable Electromagnetic Recovery Tool—C Free Point Indicator Tool Free Point Indicator Tool—A Free Point Indicator Tool—C (monocable) Free Point Indicator Tool—D Free Point Indicator Tool—E (monocable) GeoSteering Tool GeoSteering Tool—CBB Inclination at Bit Tool Inclination at Bit Tool 4.75≤ Inclination at Bit Tool—AA 4.75≤ Inclination at Bit Tool—AB 6.75≤ Inclination at Bit Tool—AA 8.0≤ Inclination at Bit Tool—AA 9.625≤ Inclination at Bit Tool—AA Pseudo-tool for IDEAL Surface Data Acquisition M10 Navigational Sub (PowerPulse) Measurement While Drilling Slim Hole Retrievable MWD Tool Slim Hole Retrievable MWD Tool VIPER Slimhole Coiled Tubing MWD Tool—AA VIPER Slimhole Coiled Tubing MWD Tool General AMS AST BGIC BGS BGT BGTC BGTX BHTV BTTA BTTB Auxiliary Measurement Sonde Acoustic Scanner Tool Borehole Geometry Interface Cartridge Borehole Geometry Sonde Borehole Geometry Tool Borehole Geometry Tool Borehole Geometry Tool Borehole Televiewer Borehole Televiewer—A Borehole Televiewer—B
    • 275 Contractor Tool Mnemonics Mnemonic Type Mode Application BTTC CALI CBTT CMT ECD ECDC EDAC EMS EMSA EMSB GCAD BHTV CALI BHTV CMT ECD ECD EDAC EMS EMS EMS GCAD WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Scanning Caliper Scanning Scanning Caliper Caliper Caliper Auxiliary Auxiliary Auxiliary Geometry GCADA GCADB GCADC GCT GCTA GCTAB GCTB GCTBB GPIT GPITB HRCC GCAD GCAD GCAD GCT GCT GCT GCT GCT GPIT GPIT HRCC WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Geometry Geometry Geometry Geometry Geometry Geometry Geometry Geometry Geometry Geometry Caliper MCD MCDB MCDD MCDF MCDG NOSE NOSEA SBTTA SPCS TCS TCSC TCSE TCSX UBI VCD VCDD MCD MCD MCD MCD MCD NOSE NOSE BHTV SPCS TCS TCS TCS TCS UBI VCD VCD WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Caliper Caliper Caliper Caliper Caliper Auxiliary Auxiliary Scanning Caliper Caliper Caliper Caliper Caliper Scanning Caliper Caliper FBST FBSTA FBSTB GHMA FBST FBST FBST GHMT WIRELINE WIRELINE WIRELINE WIRELINE Scanning Scanning Scanning Geomagnetism GHMT GHMT WIRELINE Geomagnetism HDT HDT WIRELINE Dipmeter Description Borehole Televiewer—C Generalized Caliper Combinable Borehole Televiewer Circumferential Microsonic Tool Eccentered Caliper Device Eccentered Caliper Device—C Eccentered Dual Axis Caliper Environment Measurement Sonde Environmental Measurement Sonde—A Environmental Measurement Sonde—B Guidance Continuous Tool Anchor Device GCT Anchoring Device—A GCT Anchoring Device—B GCT Anchoring Device—C Guidance Continuous Tool Guidance Continuous Tool—A Guidance Continuous Tool—AB Guidance Continuous Tool—B Guidance Continuous Tool—BB General Inclinometry Tool General Purpose Inclinometry Tool HILT High Resolution Common Cartridge Mechanical Caliper Device Mechanical Caliper Device—B Mechanical Caliper Device—D Mechanical Caliper Device—F Mechanical Caliper Device—G Nose Orienting Scanning Equipment Nose Orienting Scanning Equipment—A Slim Hole Borehole Televiewer—A Slim Powered Caliper Sonde Through Tubing Caliper Sonde Through Tubing Caliper Sonde—C Through Tubing Caliper Sonde—E Through Tubing Caliper Sonde—X Ultrasonic Borehole Imager Caliper Device Caliper Device Geology Full Bore Scanner Tool Full-Bore Scanner—A Full-Bore Scanner—B Geological High Sensitivity Magnetic Tool A Geological High Sensitivity Magnetic Tool High Resolution Dipmeter Tool
    • 276 Mnemonic Well Logging and Formation Evaluation Type Mode Application HDTD HDTE HDTF HDTG HDTJ MESC MEST MESTA MESTB MESTC OBDT OBDTA OBDTAB OBDTB OBMT OBMT-AA HDT HDT HDT HDT HDT MEST MEST MEST MEST MEST OBDT OBDT OBDT OBDT OBMT OBMT WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Dipmeter Dipmeter Dipmeter Dipmeter Dipmeter Scanning Scanning Scanning Scanning Scanning Dipmeter Dipmeter Dipmeter Dipmeter Scanning Scanning OBMT-AB OBMT WIRELINE Scanning OBMT-ABA OBMT WIRELINE Scanning OBMT-ABB OBMT WIRELINE Scanning OBMT-B OBMT WIRELINE Scanning SHDT SHDT WIRELINE Dipmeter SHDTA SHDT WIRELINE Dipmeter SHDTB SHDT WIRELINE Dipmeter CSAT CSAT1 CSAT2 CSAT3 CSAT4 CWRT CSAT CSAT CSAT CSAT CSAT CWRT WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Acoustic Acoustic Acoustic Acoustic Acoustic Acoustic CWRTA DSA DSAA DSAB QSST QSSTB SAT SATA SATB VSIT CWRT DSA DSA DSA QSST QSST SAT SAT SAT VSIT WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Acoustic Acoustic Acoustic Acoustic Acoustic Acoustic Acoustic Acoustic Acoustic Acoustic Description High Resolution Dipmeter Tool—D High Resolution Dipmeter Tool—E High Resolution Dipmeter Tool—F High Resolution Dipmeter Tool—G High Resolution Dipmeter Tool—J Micro-Electrical Scanner Cartridge Micro-Electrical Scanner Tool Micro-Electrical Scanner Tool—A Micro-Electrical Scanner Tool—B Micro-Electrical Scanner Tool—C Oil Base Mud Dipmeter Tool Oil Base Mud Dipmeter Tool—A Oil Base Mud Dipmeter Tool—AB Oil Base Mud Dipmeter Tool—B Oil Base Mud Formation Imager Tool Oil Base Mud Formation Imager Tool—AA Oil Base Mud Formation Imager Tool—AB Oil Base Mud Formation Imager Tool—ABA Oil Base Mud Formation Imager Tool—ABB Oil Base Mud Formation Imager Tool—B Stratigraphic High Resolution Dipmeter Tool Stratigraphic High Resolution Dipmeter Tool Stratigraphic High Resolution Dipmeter Tool Geophysics Seismic Acquisition Tool Combined Seismic Acquisition Tool 1 Combined Seismic Acquisition Tool 2 Combined Seismic Acquisition Tool 3 Combined Seismic Acquisition Tool 4 Cross Well Receiver Tool, used to receive cross-well seismic signals Cross Well Receiver Tool—A Downhole Seismic Array Downhole Seismic Array—A Downhole Seismic Array—B Quick Shot Seismic Tool Quick Shot Seismic Tool—B Seismic Acquisition Tool Seismic Acquisition Tool—A Seismic Acquisition Tool—B Versatile Seismic Imager
    • 277 Contractor Tool Mnemonics Mnemonic Type Mode Application VSIT-A WSAM VSIT WSAM WIRELINE WIRELINE Acoustic Acoustic WST WSTA WST WST WIRELINE WIRELINE Acoustic Acoustic AACT ACTC ACTD AND ADN4AA AACT AACT AACT ADN ADN WIRELINE WIRELINE WIRELINE MWD MWD Nuclear Nuclear Nuclear Nuclear Nuclear ADN675 ADN MWD Nuclear ADN6AA ADN MWD Nuclear ADN6BA ADN MWD Nuclear AGS AGS WIRELINE Nuclear AGS-AA AGS WIRELINE Nuclear AGS-BA AGS WIRELINE Nuclear AGS_AA AGS WIRELINE Nuclear AGS_BA AGS WIRELINE Nuclear AIT AITB AITC AITH AITS ALAT ALATA ALATB APS APS-AA APS-BA AIT AIT AIT AIT AIT ALAT ALAT ALAT APS APS APS WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Resistivity Resistivity Resistivity Resistivity Resistivity Resistivity Resistivity Resistivity Nuclear Nuclear Nuclear APS-C ARC APS ARC WIRELINE MWD Nuclear Resistivity ARC5AA ARC MWD Resistivity Description Versatile Seismic Imager Well Seismic Acquisition Surface Module Well Seismic Tool Seismic Acquisition Tool Petrophysics Aluminium Activation Clay Tool Aluminium Activation Clay Tool—C Aluminium Activation Clay Tool—D Azimuthal Density Neutron Tool 4.75 Inch Azimuthal Density Neutron Tool Azimuthal Density Neutron Tool, 6.75 inch 6.75 Inch Azimuthal Density Neutron Tool 6.75 Inch Azimuthal Density Neutron Tool Aluminum Gamma Ray Spectroscopy Sonde (same hardware as HNGS) Aluminum Gamma Ray Spectroscopy Sonde—AA (same hardware as HNGS-AA) Aluminum Gamma Ray Spectroscopy Sonde—BA (same hardware as HNGS-BA) Aluminum Gamma Ray Spectroscopy Sonde—AA (same hardware as HNGS-AA) Aluminum Gamma Ray Spectroscopy Sonde—BA (same hardware as HNGS-BA) Array Induction Imager Tool Array Induction Tool—B Array Induction Tool—C Array Induction Tool—H Array Induction Tool Slimhole Azimuthal Laterolog Azimuthal Laterolog—A Azimuthal Laterolog—B Accelerator Porosity Sonde Accelerator Porosity Sonde—AA Accelerator Porosity Sonde—BA (both CTS and DTS Telemetry) Accelerator Porosity Sonde—C Array Compensated Resistivity— Gamma Ray Tool 4.75 Inch Array Resistivity Compensated Tool—AA
    • 278 Mnemonic Well Logging and Formation Evaluation Type Mode Application ARC5AB ARC MWD Resistivity ARC5BA ARC MWD Resistivity BSP CDM6AA SP CDR WIRELINE MWD Spontaneous Resistivity CDM6AB CDR MWD Resistivity CDM8AA CDR MWD Resistivity CDM8AB CDR MWD Resistivity CDN CDN650 CDN CDN MWD MWD Nuclear Nuclear CDN800 CDN MWD Nuclear CDR CDR MWD Resistivity CDR475 CDR MWD Resistivity CDR650 CDR MWD Resistivity CDR675 CDR MWD Resistivity CDR800 CDR MWD Resistivity CDR825 CDR MWD Resistivity CDR950 CDR MWD Resistivity CFRT CFRT-C CGRS CMR CMR-A CFRT CFRT CGRS CMR CMR WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Resistivity Resistivity Nuclear Nuclear_Magnetic Nuclear_Magnetic CMR-B CMR WIRELINE Nuclear_Magnetic CMRT CNT CNTA CNTD CNTE CNTF CNTG CNTH CNTS CSA1 CMR CNT CNT CNT CNT CNT CNT CNT CNT CST WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Nuclear_Magnetic Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Sampling Description 4.75 Inch Array Resistivity Compensated Tool—AB 4.75 Inch Array Resistivity Compensated Tool—BA Potential Bridle 6.75 Inch Compensated Dual Resistivity Tool 6.75 Inch Compensated Dual Resistivity Tool 8.25 Inch Compensated Dual Resistivity Tool 8.25 Inch Compensated Dual Resistivity Tool Compensated Density Neutron Tool Compensated Density Neutron Tool, 6.5 inch Compensated Density Neutron Tool, 8.0 inch Compensated Dual Resistivity—Gamma Ray Tool Compensated Dual Resistivity—Gamma Ray Tool, 4.75 inch Compensated Dual Resistivity—Gamma Ray Tool, 6.5 inch Compensated Dual Resistivity—Gamma Ray Tool, 6.75 inch Compensated Dual Resistivity—Gamma Ray Tool, 8.0 inch Compensated Dual Resistivity—Gamma Ray Tool, 8.25 inch Compensated Dual Resistivity—Gamma Ray Tool, 9.5 inch Cased-hole Formation Resistivity Cased-hole Formation Resistivity—C Compact Gamma Ray Sonde Combinable Magnetic Resonance Tool Combinable Magnetic Resonance Tool—A Combinable Magnetic Resonance Tool—B Combinable Magnetic Resonance Tool Compensated Neutron Tool Compensated Neutron Tool—A Compensated Neutron Tool—D Compensated Neutron Tool—E Compensated Neutron Tool—F Compensated Neutron Tool—G Compensated Neutron Tool—H Compensated Neutron Tool—S Bottom Gun Modified CST-A
    • 279 Contractor Tool Mnemonics Mnemonic CSG2 CST CSTA CSTAA CSTB CSTBA CSTC CSTDA CSTG CSTG2 CSTJ CSTU CSTV CSTW CSTY CSTZ CSX1 CSX2 CSX3 CSZ1 CSZ2 CSZ3 DIT DITB DITD DITE DITX DLT DLTB DLTC DLTD DLTE DPT DPTA DPTB DSLC DSLT DSLT-BA DSLT-BB DSLT-BC DSLT-H DSLTBA DSLTBB DSLTBC DSST DSST-C DSSTA DSSTB DSSTC DST Type CST CST CST CST CST CST CST CST CST CST CST CST CST CST CST CST CST CST CST CST CST CST DIT DIT DIT DIT DIT DLT DLT DLT DLT DLT DPT DPT DPT DSLT DSLT DSLT DSLT DSLT DSLT DSLT DSLT DSLT DSST DSST DSST DSST DSST DST Mode WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Application Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling Resistivity Resistivity Resistivity Resistivity Resistivity Resistivity Resistivity Resistivity Resistivity Resistivity Dielectric Dielectric Dielectric Acoustic Acoustic Acoustic Acoustic Acoustic Acoustic Acoustic Acoustic Acoustic Acoustic Acoustic Acoustic Acoustic Acoustic Resistivity Description Core Sample Taker Core Sample Taker Core Sample Taker Core Sample Taker Core Sample Taker Core Sample Taker—BA Core Sample Taker Core Sample Taker—DA Core Sample Taker Core Sample Taker Core Sample Taker—J Core Sample Taker Core Sample Taker Core Sample Taker Core Sample Taker Core Sample Taker Bottom Gun Modified CST-G Middle Gun Modified CST-G Top Gun Modified CST-G Bottom Gun Modified CST-Z Middle Gun Modified CST-Z Upper Gun Modified CST-Z Dual Induction Tool Dual Induction Tool—B Dual Induction Tool—D Dual Induction Tool (Phasor) Dual Induction Tool—D (with BGIC) Dual Laterolog Tool Dual Laterolog Tool—B Dual Laterolog Tool—C Dual Laterolog Tool—D Dual Laterolog Tool—E Dielectric Propagation Tool Dielectric Propagation Tool Dielectric Propagation Tool Digitizing Sonic Logging Cartridge Digitizing Sonic Logging Tool Digitizing Sonic Logging Tool—BA Digitizing Sonic Logging Tool—BB Digitizing Sonic Logging Tool—BC Digitizing Sonic Logging Tool—H Digitizing Sonic Logging Tool—BA Digitizing Sonic Logging Tool—BB Digitizing Sonic Logging Tool—BC Dipole Shear Sonic Imager Tool Dipole Shear Sonic Imager Tool—C Dipole Shear Sonic Imager Tool—A Dipole Shear Sonic Imager Tool—B Dipole Shear Sonic Imager Tool—C Dual Laterolog Tool with SRT
    • 280 Well Logging and Formation Evaluation Mnemonic Type Mode Application DSTB DSTD DSTE DWST ECC DST DST DST DWST ECC WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Resistivity Resistivity Resistivity Acoustic Nuclear ECC-A ECC WIRELINE Nuclear ECS ECS-A ECS ECS WIRELINE WIRELINE Nuclear Nuclear ECSB ECSC WIRELINE Nuclear ECSC ECSC WIRELINE Nuclear ECSC-AA ECSC WIRELINE Nuclear ECSC-BA ECSC WIRELINE Nuclear EPT EPTD EPTE EPTG ES FGT FGTC FGTCA GNT GNTK GNTN GRA GRT GRTC EPT EPT EPT EPT ES FGT FGT FGT GNT GNT GNT GRA GRT GRT WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Dielectric Dielectric Dielectric Dielectric Resistivity Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear GST GST-A HALS HALS-BHALS HALSB HAPS-BA HAPS-C HGNS HILT GST GST HALS HALS HALS APS APS HGNS HILT WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Nuclear Nuclear Resistivity Resistivity Resistivity Nuclear Nuclear Nuclear Combination HILTB HILT WIRELINE Combination HILTC HILT WIRELINE Combination HILTD HILT WIRELINE Combination Description Dual Laterolog Tool with SRT Dual Laterolog Tool with SRT Dual Laterolog Tool with SRT Digital Waveform Sonic Tool Elemental Capture Spectroscopy Cartridge (supports ECS) Elemental Capture Spectroscopy Cartridge—A Elemental Capture Spectroscopy Sonde Elemental Capture Spectroscopy Sonde—A Elemental Capture Spectroscopy Cartridge—BA, DTS(FTB) Telemetry Elemental Capture Spectroscopy Cartridge (same hardware as NPLC) Elemental Capture Spectroscopy Cartridge—AA, CTS(DTB) Telemetry Elemental Capture Spectroscopy Cartridge—BA, DTS(FTB) Telemetry Electromagnetic Propagation Tool Electromagnetic Propagation Tool—D Electromagnetic Propagation Tool—E Electromagnetic Propagation Tool—G Electrical Survey Tool Formation Gamma Gamma Tool Formation Gamma Gamma Tool—C Formation Gamma Gamma Tool—CA Gamma Neutron Tool Gamma Neutron Tool Gamma Neutron Tool Geochemical Reservoir Analyzer Gamma Ray Tool High Temperature Gamma Ray Tool, 1–3/8 Inch Gamma Spectroscopy Tool Gamma Spectroscopy Tool HILT Azimuthal Laterolog Sonde HILT Azimuthal Laterolog Sonde—B HILT Azimuthal Laterolog Sonde—B HPHT Accelerator Porosity Sonde— HPHT Accelerator HILT Gamma-Ray Neutron Sonde High Resolution Integrated Logging Tool High Resolution Integrated Logging Tool—B High Resolution Integrated Logging Tool—CTS Stand-alone High Resolution Integrated Logging Tool—DTS Combinable
    • 281 Contractor Tool Mnemonics Mnemonic Type Mode Application HIT HIT-A HITA HLDS HLDT HLDTA HIT HIT HIT HLDS HLDT HLDT WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Resistivity Resistivity Resistivity Nuclear Nuclear Nuclear HNCC HNCC-A HNCC HNCC WIRELINE WIRELINE Nuclear Nuclear HNGC HNGC WIRELINE Nuclear HNGC-A HNGC WIRELINE Nuclear HNGS HNGS WIRELINE Nuclear HNGS-AA HNGS WIRELINE Nuclear HNGS-BA HNGS WIRELINE Nuclear HNGT HNGT WIRELINE Nuclear HNPL-BA NPLC WIRELINE Nuclear HRDD HRGD HRDD HRGD WIRELINE WIRELINE Nuclear Combination HRLA HRLA-A HRLT HRLT-B HRLA HRLA HRLT HRLT WIRELINE WIRELINE WIRELINE WIRELINE Resistivity Resistivity Resistivity Resistivity HRLT-C HRLT WIRELINE Resistivity HSGT HSGTA HSGT HSGT WIRELINE WIRELINE Nuclear Nuclear HSLT HSLT-A ILTA IMPA IMPA-AA DSLT DSLT IRT IMPA IMPA WIRELINE WIRELINE WIRELINE MWD MWD Acoustic Acoustic Resistivity Resistivity Resistivity IMPA-AB IMPA MWD Resistivity IMPA-BA IMPA MWD Resistivity IRT IRTF IRTJ IRT IRT IRT WIRELINE WIRELINE WIRELINE Resistivity Resistivity Resistivity Description Hostile Array Induction Tool Hostile Array Induction Tool—A Hostile Array Induction Tool—A Hostile Litho-Density Sonde Hostile Environment Litho Density Tool Hostile Environment Litho Density Tool—A Hostile Environment Nuclear Cartridge Hostile Environment Nuclear Cartridge—A Hostile Natural Gamma Ray Spectrometry Cartridge Hostile Natural Gamma Ray Spectrometry Cartridge— Hostile Natural Gamma Ray Spectrometry Sonde Hostile Natural Gamma Ray Sonde—AA Hostile Natural Gamma Ray Sonde—BA Hostile Natural Gamma Ray Spectrometry Tool HPHT Nuclear Porosity Lithology Cartridge HILT High Resolution Density Device HILT High Resolution Resistivity Gamma-Ray Density High Resolution Laterolog Array High Resolution Laterolog Array—A High Resolution Laterolog Array Tool High Resolution Laterolog Array Tool—B High Resolution Laterolog Array Tool—C Hostile Environment Gamma Ray Tool Hostile Environment Gamma Ray Tool—A HPHT Sonic Logging Tool HPHT Digitizing Sonic Logging Tool Induction Logging Tool Compensated Array Resistivity Tool 4.75 Inch Prototype Compensated Array Resistivity 4.75 Inch Production Compensated Array Resistivity Tool without IAB 4.75 Inch Production Compensated Array Resistivity Tool with IAB Induction Resistivity Tool Induction Resistivity Tool Induction Resistivity Tool
    • 282 Well Logging and Formation Evaluation Mnemonic Type Mode Application IRTK IRTL IRTM IRTN IRTQ IRTR IRTX IRT IRT IRT IRT IRT IRT IRT WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Resistivity Resistivity Resistivity Resistivity Resistivity Resistivity Resistivity ISONIC LDS LDSC ISONIC LDS LDSC MWD WIRELINE WIRELINE Acoustic Nuclear Nuclear LDSC-A LDT LDTA LDTC LDTD LL3 LL7 MCFL LDSC LDT LDT LDT LDT LL3 LL7 MCFL WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Nuclear Nuclear Nuclear Nuclear Nuclear Resistivity Resistivity Resistivity MDLT MDLTA MDST MDSTA MIST MISTA MISTB MLL MDLT MDLT MDST MDST MIST MIST MIST MLL WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Resistivity Resistivity Resistivity Resistivity Nuclear Nuclear Nuclear Resistivity MLT MLTA MLTAA MPT MPTD MRWD MRWD6-AA MLT MLT MLT MPT MPT MRWD MRWD WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE MWD MWD Resistivity Resistivity Resistivity Resistivity Resistivity Nuclear_Magnetic Nuclear_Magnetic MSCT MSCTA MSGT MSCT MSCT MSGT WIRELINE WIRELINE WIRELINE Sampling Sampling Nuclear MSGTA NDS NDS6AA MSGT CDN CDN WIRELINE MWD MWD Nuclear Nuclear Nuclear NDS6AB CDN MWD Nuclear NDS6BA CDN MWD Nuclear Description Induction Resistivity Tool Induction Resistivity Tool Induction Resistivity Tool Induction Resistivity Tool Induction Resistivity Tool Induction Resistivity Tool Induction Resistivity Tool—Q (with BGIC) LWD Sonic Tool Litho Density Sonde (for IPLT) Litho Density Cartridge (supports LDS or HLDS) Litho Density Cartridge—A Litho Density Tool Litho Density Tool—A Litho Density Tool—C Litho Density Tool—D Laterolog 3 Tool (predates digital era) Laterolog 7 Sonde (predates digital era) Micro-Cylindrically Focused Logging Device Medium Dual Laterolog Tool Medium Dual Laterolog Tool—A Medium Dual Laterolog SFL Tool Medium Dual Laterolog SFL Tool—A Multiple Isotope Spectroscopy Tool Multiple Isotope Spectroscopy Tool—A Multiple Isotope Spectroscopy Tool—B Microlaterologlog Tool (predates digital era) Microlog Tool Microlog Tool Microlog Tool—AA Microlog Proximity Tool Microlog Proximity Tool—D Magnetic Resonance While Drilling Magnetic Resonance While Drilling—AA Mechanical Sidewall Coring Tool Mechanical Sidewall Coring Tool—A Scintillation Gamma Ray Tool, 2–3/4 inch Scintillation Gamma Ray Tool—A Neutron Density Sonde 6.5 Inch Low Flow Compensated Density Neutron Tool 6.5 Inch Low Flow Compensated Density Neutron Tool 6.5 Inch High Flow Compensated Density Neutron Tool
    • 283 Contractor Tool Mnemonics Mnemonic Type Mode Application NDS8AA CDN MWD Nuclear NDS8AB CDN MWD Nuclear NGS NGS WIRELINE Nuclear NGSA NGS WIRELINE Nuclear NGSB NGS WIRELINE Nuclear NGT NGTA NGT NGT WIRELINE WIRELINE Nuclear Nuclear NGTB NGT WIRELINE Nuclear NGTC NGT WIRELINE Nuclear NGTD NGT WIRELINE Nuclear NGTE NGT WIRELINE Nuclear NGTF NGT WIRELINE Nuclear NMT NMTC NMTCA NMTCB NPLC NPLC-AA NMT NMT NMT NMT NPLC NPLC WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Nuclear_Magnetic Nuclear_Magnetic Nuclear_Magnetic Nuclear_Magnetic Nuclear Nuclear NPLC-BA NPLC WIRELINE Nuclear NPLC-BB NPLC WIRELINE Nuclear NPLT PCD PCDA PCDB PGT PGTE PGTF PGTG PGTH PGTK PGTL PGTM PNT PNTA PNTB NPLT PCD PCD PCD PGT PGT PGT PGT PGT PGT PGT PGT PNT PNT PNT WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Nuclear Resistivity Resistivity Resistivity Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Description 8.0 Inch Compensated Density Neutron Tool 8.0 Inch Compensated Density Neutron Tool Natural Gamma Ray Spectrometry Sonde Natural Gamma Ray Spectrometry Sonde Natural Gamma Ray Spectrometry Sonde Natural Gamma Ray Spectrometry Tool Natural Gamma Ray Spectrometry Tool—A Natural Gamma Ray Spectrometry Tool—B Natural Gamma Ray Spectrometry Tool—C Natural Gamma Ray Spectrometry Tool—D Natural Gamma Ray Spectrometry Tool—E Natural Gamma Ray Spectrometry Tool—F Nuclear Magnetism Tool Nuclear Magnetism Tool—C Nuclear Magnetism Tool—CA Nuclear Magnetism Tool—CB Nuclear Porosity Lithology Cartridge Nuclear Porosity Lithology Cartridge— AA (CTS Telemetry) Nuclear Porosity Lithology Cartridge— BA (DTS Telemetry) Nuclear Porosity Lithology Cartridge— BB (DTS Telemetry, w/o HNGS board) Nuclear Porosity Lithology Tool Powered Caliper Device Powered Caliper Device—A Powered Caliper Device—B Formation Density Tool Compensated Density Tool Compensated Density Tool Compensated Density Tool Compensated Density Tool Compensated Density Tool Compensated Density Tool Compensated Density Tool Sidewall Neutron Tool Sidewall Neutron Tool—A Sidewall Neutron Tool—B
    • 284 Mnemonic Well Logging and Formation Evaluation Type Mode Application PNTC QAIT QCNT-A QLDTA RAB RAB675 PNT HIT CNT SLDT RAB RAB WIRELINE WIRELINE WIRELINE WIRELINE MWD MWD Nuclear Resistivity Nuclear Nuclear Resistivity Resistivity RAB6AA RAB6B RAB825 RAB RAB RAB MWD MWD MWD Resistivity Resistivity Resistivity RAB8A RGM8A RAB CDR MWD MWD Resistivity Resistivity RGM8A CDR MWD Resistivity RGM8AC CDR MWD Resistivity RGM9AA CDR MWD Resistivity RGM9AB CDR MWD Resistivity RGS CDR MWD Resistivity RGS6AA CDR MWD Resistivity RST RSTA RSTB SAIT SAIT-AA SDT SDTA SDTB SDTC SDTE SGT SGTE SGTEA SGTEE SGTFL SGTG SGTK SGTL SGTN SGTR SLDT SLDTA SLDTB RST RST RST SAIT SAIT SDT SDT SDT SDT SDT SGT SGT SGT SGT SGT SGT SGT SGT SGT SGT SLDT SLDT SLDT WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Nuclear Nuclear Nuclear Resistivity Resistivity Acoustic Acoustic Acoustic Acoustic Acoustic Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Description Sidewall Neutron Tool—C Slim Hostile Array Induction Tool—A SlimHot Compensated Neutron Tool—A SlimHot Litho-Density Tool—A Azimuthal Laterolog—Gamma Ray Tool Azimuthal Laterolog—Gamma Ray Tool, 6.75 inch 6.75 Inch Resistivity At the Bit Tool 6.75 Inch Resistivity At the Bit Tool Azimuthal Laterolog—Gamma Ray Tool, 8.25 inch 8.25 Inch Resistivity At the Bit Tool 8.0 Inch Compensated Dual Resistivity Tool 8.0 Inch Compensated Dual Resistivity Tool 8.0 Inch Compensated Dual Resistivity Tool 9.5 Inch Compensated Dual Resistivity Tool 9.5 Inch Compensated Dual Resistivity Tool Compensated Dual Resistivity—Gamma Ray 6.5 Inch Compensated Dual Resistivity Tool Reservoir Saturation Tool Reservoir Saturation Tool—A Reservoir Saturation Tool—B Slimhole Array Induction Tool Slimhole Array Induction Tool Sonic Digital Tool Sonic Digital Tool—A Sonic Digital Tool—B Sonic Digital Tool—C Sonic Digital Tool—E Scintillation Gamma Ray Tool Scintillation Gamma Ray Tool Scintillation Gamma Ray Tool—EA Scintillation Gamma Ray Tool—EE Scintillation Gamma-Ray Scintillation Gamma Ray Tool—G Scintillation Gamma Ray Tool—K Scintillation Gamma Ray Tool—L Scintillation Gamma Ray Tool—N Scintillation Gamma Ray Tool—R Slimhole Litho-Density Tool Slimhole Litho-Density Tool—A Slimhole Litho-Density Tool (Hostile Environment)
    • 285 Contractor Tool Mnemonics Mnemonic Type Mode Application SLT SLTJ SLTL SLTM SLTN SLTQ SLTS SLTT SMRT SMRTA SNPD SON675 SON825 SP SPA SPAA SPE SPEA SPIN SLT SLT SLT SLT SLT SLT SLT SLT SMRT SMRT SNPD ISONIC ISONIC SP SPA SPA SPE SPE SP WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE MWD MWD WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Acoustic Acoustic Acoustic Acoustic Acoustic Acoustic Acoustic Acoustic Resistivity Resistivity Nuclear Acoustic Acoustic Potential Potential Potential Potential Potential Potential SRT SRT WIRELINE Resistivity SRTB SRT WIRELINE Resistivity SRTC SRT WIRELINE Resistivity SRTD SRT WIRELINE Resistivity SRTX SRT WIRELINE Resistivity SSGT SSGTA SSLT SSLTA SSLTAA STCB SWD8AA SSGT SSGT SSLT SSLT SSLT CST ISONIC MWD WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE LWD Nuclear Nuclear Acoustic Acoustic Acoustic Sampling Acoustic SWD8BA ISONIC MWD LWD Acoustic SWD8CA ISONIC MWD LWD Acoustic SWT SWTC SWTCA SWTS SWTSA SWTSB SWTX SWTXA SWT SWTC SWTC SWTS SWTS SWTS SWTX SWTX Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear Nuclear WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Description Sonic Logging Tool Sonic Logging Tool Sonic Logging Tool Sonic Logging Tool Sonic Logging Tool Sonic Logging Tool Sonic Logging Tool Sonic Logging Tool Slim Micro Resistivity Tool Slim Micro Resistivity Tool—A Sidewall Neutron Tool Sonic Tool, 6.75 inch Sonic Tool, 8.25 inch Spontaneous Potential Spontaneous_Potential SP Adapter Spontaneous_Potential SP Adapter—A Spontaneous_Potential SP Extender Spontaneous_Potential SP Extender—A Spontaneous_Potential Dummy Tool for SP Micro Spherically Focused Resistivity Tool Micro Spherically Focused Resistivity Tool—B Micro Spherically Focused Resistivity Tool—C Micro Spherically Focused Resistivity Tool—D Micro Spherically Focused Resistivity Tool—X Scintillation Gamma Ray Tool Scintillation Gamma Ray Tool—A SLIM Sonic Logging Tool SLIM Sonic Logging Tool—A SLIM Sonic Logging Tool—AA Core Sample Taker 8.25 Inch Engineering Prototype Sonic While Drilling Tool 8.25 Inch Experimental Monopole Sonic While Drilling Tool 8.25 Inch Pilot Series Sonic While Drilling Tool Water Saturation Tool Water Saturation Tool Cartridge Water Saturation Tool Cartridge—A Water Saturation Tool Sonde Water Saturation Tool Sonde—A Water Saturation Tool Sonde—B Water Saturation Equipment Water Saturation Equipment—A
    • 286 Well Logging and Formation Evaluation Mnemonic TSGT Type SGT Mode Application Description WIRELINE Nuclear Gamma Ray Tool Production Acoustic Spectrum Measuring Tool Bottom Sand Detection Tool CPLT Flowmeter (may be a model of FBS or CFS) CPLT Flowmeter 1 (may be a model of FBS or CFS) CPLT Flowmeter 2 (may be a model of FBS or CFS) Continuous Flowmeter Sonde Continuous Flowmeter Sonde—F Continuous Flowmeter Sonde—H Continuous Flowmeter Sonde—J Continuous Flowmeter Sonde—K Continuous Flowmeter Sonde—N Continuous Flowmeter Sonde—P Continuous Flowmeter Sonde—Q Continuous Flowmeter Sonde—R Continuous Flowmeter Sonde—X CPLT HP Gauge Compact Production Logging Cartridge Compact Production Logging Sonde CTS Production Logging Tool CTS Production Logging Tool—A CTS Production Logging Tool—B CTS Production Logging Tool—C Pressure Gauge Tool (FlopetrolJohnston) Digital Entry and Fluid Imager Tool Digital Entry and Fluid Imager Tool—A Digital Entry and Fluid Imager Tool—AB Digital Entry and Fluid Imager Tool— AB (second DEFT-AB in toolstring) Digital Entry and Fluid Imager Tool—A (second DEFT-A in toolstring) Digital Entry and Fluid Imager Tool—B Digital Entry and Fluid Imager Tool—C Digital Entry and Fluid Imager Tool—C (second DEFT-C in toolstring) Dual Flowmeter Interface Cartridge Dual Flowmeter Interface Cartridge—A Dual Flowmeter Interface Cartridge—B Differential Gravity Tool Differential Gravity Tool—AA Electrical Flowmeter Tool (FlopetrolJohnston) External Surface Pressure and Temperature Measurements Production Logging ASMT BSDT CFM ASMT BSDT CFM WIRELINE WIRELINE WIRELINE Acoustic Acoustic Flowmeter CFM1 CFM WIRELINE Flowmeter CFM2 CFM WIRELINE Flowmeter CFS CFSF CFSH CFSJ CFSK CFSN CFSP CFSQ CFSR CFSX CHMS CPLC CPLS CPLT CPLTA CPLTB CPLTC CRG CFS CFS CFS CFS CFS CFS CFS CFS CFS CFS CHMS CPLC CPLS CPLT CPLT CPLT CPLT CRG WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Flowmeter Flowmeter Flowmeter Flowmeter Flowmeter Flowmeter Flowmeter Flowmeter Flowmeter Flowmeter Pressure Combination Combination Combination Combination Combination Combination Pressure DEFT DEFTA DEFTAB DEFTAB_2 DEFT DEFT DEFT DEFT WIRELINE WIRELINE WIRELINE WIRELINE Flowmeter Flowmeter Flowmeter Flowmeter DEFTA_2 DEFT WIRELINE Flowmeter DEFTB DEFTC DEFTC_2 DEFT DEFT DEFT WIRELINE WIRELINE WIRELINE Flowmeter Flowmeter Flowmeter DFIC DFICA DFICB DGT DGT-AA EFM DFIC DFIC DFIC DGT DGT EFM WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Flowmeter Flowmeter Flowmeter Gravity Gravity Flowmeter EXP EXP WIRELINE Combination
    • 287 Contractor Tool Mnemonics Mnemonic Type Mode Application FBDS CFS WIRELINE Flowmeter FBDSA CFS WIRELINE Flowmeter FBSA FBSB FBSC FBSD FBSE FBSX FSMT FSMT-A CFS CFS CFS CFS CFS CFS FSMT FSMT WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Flowmeter Flowmeter Flowmeter Flowmeter Flowmeter Flowmeter Nuclear Nuclear FSMT-B FSMT WIRELINE Nuclear GHOST GHOST2 DEFT DEFT WIRELINE WIRELINE Flowmeter Flowmeter GHOSTA GHOSTA_2 DEFT DEFT WIRELINE WIRELINE Flowmeter Flowmeter GMS GMSC GMSD GPPT HCFS GMS GMS GMS GPPT HCFS WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Fluid_Density Fluid_Density Fluid_Density Nuclear Flowmeter HCFSA HCFS WIRELINE Flowmeter HCFT HCFTA HMS HMSA HMSB HPA HPAA HPAB HPXA HTT HTTA HTTB HTTC HTTCA HUM HUMA HUMB ISDT HCFT HCFT HMS HMS HMS HPA HPA HPA HPA HTT HTT HTT HTT HTT HUM HUM HUM ISDT WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Flowmeter Flowmeter Pressure Pressure Pressure Pressure Pressure Pressure Pressure Temperature Temperature Temperature Temperature Temperature Fluid_Density Fluid_Density Fluid_Density Acoustic LEE_FM LEE_FM WIRELINE Flowmeter Description Full Bore Directional Spinner Flowmeter Sonde Full Bore Directional Spinner Flowmeter Sonde—A Full Bore Spinner Flowmeter Sonde—A Full Bore Spinner Flowmeter Sonde—B Full Bore Spinner Flowmeter Sonde—C Full Bore Spinner Flowmeter Sonde—D Full Bore Spinner Flowmeter Sonde—E Full Bore Spinner Flowmeter Sonde—X Formation Subsidence Monitoring Tool Formation Subsidence Monitoring Tool—A Formation Subsidence Monitoring Tool—B Gas Holdup Optical Sensing Tool Gas Holdup Optical Sensing Tool—A (2nd GHOST-A in tool string’) Gas Holdup Optical Sensing Tool—A Gas Holdup Optical Sensing Tool—A (2nd GHOST-A in tool string’) Gradiomanometer Sonde Gradiomanometer Sonde Gradiomanometer Sonde Gamma Neutron Tool High Temperature Continuous Flowmeter Sonde High Temperature Continuous Flowmeter Sonde—A Flowmeter Sonde Flowmeter Sonde Hewlett Packard Manometer Sonde Hewlett Packard Manometer Sonde Hewlett Packard Manometer Sonde Hewlett Packard Adaptor Hewlett Packard Adaptor Hewlett Packard Adaptor Hewlett Packard Adaptor High Resolution Thermometer Tool High Resolution Thermometer Tool—A High Resolution Thermometer Tool—B High Resolution Thermometer Tool—C High Resolution Thermometer Tool—CA Hold Up Meter Hold Up Meter—A Hold Up Meter—B Inline Sand Detection Tool (3rd Party, e.g. Fluenta Technology) Flowmeter manufactured by Lee Tools, operated by Schlumberger
    • 288 Well Logging and Formation Evaluation Mnemonic Type Mode Application LIFT LIFTB MSRT LIFT LIFT MSRT WIRELINE Flowmeter WIRELINE Flowmeter PRODUCTION Combination MSRTA MSRTB MSRTC MSRTD MTSA MTSC NFD NFDA NFDB NFDC PBFS PBFSA PBFSB PBFSC PBFT PBMS PFCS PGMC PGMC-A PGMS PILSA PMIT PMIT-A PMIT-B PPS PPSA PPSB PPT PSPT PST PSTA PSTT PSTT-A PTS PTSA PTSAB PTSB PUCS PUCS-A1 MSRT MSRT MSRT MSRT MTS MTS NFD NFD NFD NFD CFS CFS CFS CFS CFS PBMS PFCS PGMC PGMC PGMS CFS PMIT PMIT PMIT PPS PPS PPS PPT PSPT PST PST PSTT PSTT PTS PTS PTS PTS PUCS PUCS PRODUCTION PRODUCTION PRODUCTION PRODUCTION WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Combination Combination Combination Combination Combination Combination Nuclear Nuclear Nuclear Nuclear Flowmeter Flowmeter Flowmeter Flowmeter Flowmeter Combination Flowmeter Fluid_Density Fluid_Density Fluid_Density Flowmeter Casing_Inspection Casing_Inspection Casing_Inspection Testing Testing Testing Testing Combination Sampling Sampling Mechanical Mechanical Combination Combination Combination Combination Pressure Combination PUCS-A2 PUCS WIRELINE Combination PVS PVS WIRELINE Special_Purpose. Description Local Impedance Flowmeter Tool Local Impedance Flowmeter Tool Multi-Sensor Recorder Tool, includes flowmeter, pressure, and temperature sensors Multi-Sensor Recorder Tool—A Multi-Sensor Recorder Tool—B Multi-Sensor Recorder Tool—C Multi-Sensor Recorder Tool—D Manometer Thermometer Sonde—A Manometer Thermometer Sonde—C Nuclear Fluid Density Tool Nuclear Fluid Density Tool—A Nuclear Fluid Density Tool—B Nuclear Fluid Density Tool—C Petal Basket Flowmeter Sonde Petal Basket Flowmeter Sonde—A Petal Basket Flowmeter Sonde—B Petal Basket Flowmeter Sonde—C Petal Basket Flowmeter Tool PSP Basic Module Sonde PSP Flowmeter Dual Caliper Sonde PSP Gradiomanometer Carrier PSP Gradiomanometer Carrier PSP Gradiomanometer Sonde PSP Flowmeter Dual Caliper Sonde Multifinger Imaging Tool Multifinger Imaging Tool—A Multifinger Imaging Tool—B Production Packer Sonde, 1–11/16 Inch Production Packer Sonde—A Production Packer Sonde—B Production Packer Tool Production Services Logging Platform Production Fluid Sampling Tool Production Fluid Sampler Tool Production Services Tractor Tool Production Services Tractor Tool—A Pressure Temperature Sonde Pressure Temperature Sonde—A Pressure Temperature Sonde—AB Pressure Temperature Sonde—B PSP Unigage Carrier Sonde PSP Unigauge Carrier Sonde—A (first PUCS-A tool of combination) PSP Unigauge Carrier Sonde—A (second PUCS-A tool of combination) Phase Velocity Sonde: marker fluid ejector for phase velocity determination
    • 289 Contractor Tool Mnemonics Mnemonic Type Mode Application PVS-AA PVS WIRELINE Special_Purpose RCT RCTA SCTT SPG SPST SPSTA SPTS SPTSA SVFS SVFSA TDMB TDT TDTK TDTM TDTP TEMP TET TETD TETE TMT TPT RCT RCT SCTT SPG SPST SPST SPTS SPTS SVFS SVFS TDT TDT TDT TDT TDT TEMP TET TET TET TMT TPT WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Flowmeter Flowmeter Temperature Pressure Sampling Sampling Combination Combination Flowmeter Flowmeter Nuclear Nuclear Nuclear Nuclear Nuclear Temperature Special_Purpose Special_Purpose Special_Purpose Temperature Combination CP_1 MDCP WIRELINE Sampling CP_2 MDCP WIRELINE Sampling CP_3 MDCP WIRELINE Sampling DP_1 MRDP WIRELINE Sampling DP_2 MRDP WIRELINE Sampling DP_3 MRDP WIRELINE Sampling DWCS DWCS-A FC_1 DWCS DWCS MRFC PRODUCTION Special_Purpose PRODUCTION Special_Purpose WIRELINE Sampling FC_2 MRFC WIRELINE Sampling FC_3 MRFC WIRELINE Sampling GFA GFAA GFT GFTA GFA GFA GFT GFT WIRELINE WIRELINE WIRELINE WIRELINE Nuclear Nuclear Nuclear Nuclear Description Phase Velocity Sonde—AA: 1 11/16 inch marker fluid ejector Flowmeter Transmitter (Rotron type) Flowmeter Transmitter (Rotron type)—A Sidewall Contact Temperature Tool Strain Pressure Gauge Production Fluid Sampler Tool Production Fluid Sampler Tool—A Pressure Temperature Sonde Pressure Temperature Sonde—A Slim Hole Vortex Flowmeter Sonde Slim Hole Vortex Flowmeter Sonde—A Thermal Decay Time Tool Thermal Decay Time Tool Thermal Decay Time Tool Thermal Decay Time Tool Thermal Decay Time Tool Temperature Tracer Ejector Tool Tracer Ejector Tool Tracer Ejector Tool Temperature Manometer Tool Temperature and Pressure Tool (Flopetrol-Johnston) Well Testing Modular Dynamics Casing Driller Probe Module 1 Modular Dynamics Casing Driller Probe Module 2 Modular Dynamics Casing Driller Probe Module 3 Modular Formation Dynamics Tester Multi-Probe Module Modular Formation Dynamics Tester Multi-Probe Module Modular Formation Dynamics Tester Multi-Probe Module Deep Water Control System Deep Water Control System—A Modular Formation Dynamics Tester Flow Control Module Modular Formation Dynamics Tester Flow Control Module Modular Formation Dynamics Tester Flow Control Module Formation Tester Gamma Ray Detector Formation Tester Gamma Ray Detector Formation Tester Gamma Ray Formation Tester Gamma Ray
    • 290 Mnemonic Well Logging and Formation Evaluation Type Mode Application HY1 HY2 HY3 HY_1 MRTT MRTT MRTT MRHY WIRELINE WIRELINE WIRELINE WIRELINE Sampling Sampling Sampling Sampling HY_2 MRHY WIRELINE Sampling HY_3 MRHY WIRELINE Sampling MDCP MDCP WIRELINE Sampling MDT MP1 MP2 MP3 MRDP MRFA MRFC MRHY MDT MRTT MRTT MRTT MRDP MRFA MRFC MRHY WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling MRMS MRPA MRPC MRPO MRPOUD MRPS MRSC MRTT MRMS MRPA MRPC MRPO MRPOUD MRPS MRSC MRTT WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling MS_1 MS_2 MS_3 MS_4 MS_5 MTS PA MRMS MRMS MRMS MRMS MRMS MTS MRPA WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Sampling Sampling Sampling Sampling Sampling Combination Sampling PC MRPC WIRELINE Sampling PO MRPO WIRELINE Sampling POUD MRPOUD WIRELINE Sampling PP1 PP2 PP3 PQG PQG1 PQG2 PS1 MRTT MRTT MRTT PQG PQG PQG MRTT WIRELINE WIRELINE WIRELINE PRODUCTION PRODUCTION PRODUCTION WIRELINE Sampling Sampling Sampling Special_Purpose Special_Purpose Special_Purpose Sampling Description MRTT Hydraulic Module MRTT Hydraulic Module MRTT Hydraulic Module Modular Formation Dynamics Tester Hydraulic Module Modular Formation Dynamics Tester Hydraulic Module Modular Formation Dynamics Tester Hydraulic Module Modular Dynamics Casing Driller Probe Module Modular Formation Dynamics Tester MRTT Multi-Probe Module MRTT Multi-Probe Module MRTT Multi-Probe Module MDT Dual-Probe Module Optical Fluid Analyzer MDT Flow Control Module Modular Formation Dynamics Tester Hydraulic Module MDT Multisample Module MDT Dual-Packer Module MDT Power Cartridge MDT Pump-Out Module MDT Up/Down Pump-Out Module MDT Single-Probe Module MDT Sample Module Modular Reservoir Test Tool (MDT) Multi-Sample Module (MRMS) 1 Multi-Sample Module (MRMS) 2 Multi-Sample Module (MRMS) 3 Multi-Sample Module (MRMS) 4 Multi-Sample Module (MRMS) 5 Manometer Thermometer Sonde Modular Formation Dynamics Tester Packer Modular Formation Dynamics Tester Power Cartridge Modular Formation Dynamics Tester Pumpout Modular Formation Dynamics Tester Up/Down Pumpout MRTT Precision Pressure Module MRTT Precision Pressure Module MRTT Precision Pressure Module Permanent Quartz Pressure Gauge Permanent Quartz Pressure Gauge 1 Permanent Quartz Pressure Gauge 2 MRTT Single-Probe Module
    • 291 Contractor Tool Mnemonics Mnemonic Type Mode Application PS2 PS3 PS_1 MRTT MRTT MRPS WIRELINE WIRELINE WIRELINE Sampling Sampling Sampling PS_2 MRPS WIRELINE Sampling PS_3 MRPS WIRELINE Sampling RFT RFTA RFTAB RFTB RFTTN RPQS RFT RFT RFT RFT RFT RPQS WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Sampling Sampling Sampling Sampling Sampling Sampling RTBC RFT WIRELINE Sampling RTBO RFT WIRELINE Sampling RTCU RFT WIRELINE Sampling RTOU RFT WIRELINE Sampling SC1 SC_1 MRTT MRSC WIRELINE WIRELINE Sampling Sampling SPFT SPFTA SRFT SRFTA SRFTB SRFT_A SRFT_B SRFT_C SRFT_D SRFT_E SRFT_F WTPS WTPT SPFT SPFT RFT RFT RFT RFT RFT RFT RFT RFT RFT WTPS WTPT WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE WIRELINE Testing Testing Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling Sampling Pressure Pressure Description MRTT Single-Probe Module MRTT Single-Probe Module Modular Formation Dynamics Tester Single-Probe Modular Formation Dynamics Tester Single-Probe Modular Formation Dynamics Tester Single-Probe Repeat Formation Tester Repeat Formation Tester Repeat Formation Tester Repeat Formation Tester Repeat Formation Tester Repeat Formation Tester Quartz Pressure Sonde Repeat Formation Tester Cased Hole Version Repeat Formation Tester Open Hole Version Repeat Formation Tester Cased Hole Version Repeat Formation Tester Open Hole Version MRTT Sample Chamber Modular Formation Dynamics Tester Sample Chamber Slim Packer Fluid Analyzer Tool Slim Packer Fluid Analyzer Tool Slim Repeat Formation Tester Slim Repeat Formation Tester—A Slim Repeat Formation Tester—B Slim Repeat Formation Tester—A Slim Repeat Formation Tester—B Slim Repeat Formation Tester—C Slim Repeat Formation Tester—D Slim Repeat Formation Tester—E Slim Repeat Formation Tester—F Well Test Pressure Sonde Well Test Pressure Tool
    • 292 Well Logging and Formation Evaluation Halliburton Tool Mnemonics Mnemonic Tool 2AC 2ACBA 2AC 2ACCA ACCZ BATS BATS BCS BCS BCS BCS BCS BCS BCSD BCSD BHC_GR BHCS BHCS_D BHV BIP ACCZ BATS BATSAA BCSBB BCSBD BCSD BCSFA BCSHA BCSJA BCSD BCSDAA BCS BCS BCS BHV BIPAA BPOLAR BRID BRID_R BRIDG C_GR CAL CAL_BB CAL4DC CALDC2 CALDC4 CALI_1 CALI_2 CALI_3 CALI_4 CALI_5 CALI_6 CALISW CALMSF CAST CAST COSMOS DLLT DLLT DLL DDL GR CAL SLDCAL GO GO GO SDLT-A M320 M202 HSDLM-B HFDT-A M320 M123 MSFL CAST CASTAA CAST CASTBA CAST CASTXX CASTD CASTDP CAST CAST Description TWO ARM CALIPER—DDL (older software versions) TWO ARM CALIPER—DDL (older software versions) ACCELEROMETER Z BOREHOLE AUDIO TEMPERATURE TOOL BOREHOLE AUDIO TEMPERATURE TOOL BOREHOLE COMPENSATED SONIC BOREHOLE COMPENSATED SONIC DITS B/H COMPENSATED SONIC BOREHOLE COMPENSATED SONIC BOREHOLE COMPENSATED SONIC BOREHOLE COMPENSATED SONIC BOREHOLE COMPENSATED SONIC—DITS BOREHOLE COMPENSATED SONIC—DITS BHC GAMMA TOOL BOREHOLE COMPENSATED SONIC BHC SONIC WITH DIGITAL PICKS BHV COMPUTATION PANEL— DDL SUPER STACK BOTTOM ISOLATION SUB BIPLOAR PULSE TOOLS (CosMos) CABLE ELECTRODE BRIDLE RIDGID CABLE ELECTRODE BRIDLE DLL CABLE ELECTRODE BRIDLE GO GAMMA RAY TOOL—CAST CARD PANEL—XL1 CALIPER PROCESSING SLD_BB CALIPER GO 4 ARM CALIPER GO 2 ARM CALIPER GO 4 ARM CALIPER SDL CALIPER MSFL / MEL CALIPER D202 CALIPER HOSTILE SDL CALIPER HFDT CALIPER MICRO GUARD / DLLT CALIPER SWN CALIPER DDL MSFL CALIPER ACOUSTIC SCANNING TOOL: DIO #6 CIRCUMFERENCIAL ACOUSTIC SCANNING TOOL CIRCUMFERENCIAL ACOUSTIC SCANNING TOOL CIRCUMFERENCIAL ACOUSTIC SCANNING TOOL—DITS DITS ACOUSTIC SCANNING TOOL DITS CAST_D PET MODE Type Code CALIPER XL1K CALIPER XL1K AUXILIARY COMPLETION COMPLETION SONIC SONIC SONIC SONIC SONIC SONIC SONIC SONIC SONIC SONIC SONIC DUMMY TOOL RESISTIVITY PLS2 CHLS XL1K XL1K XL1K PLS2 XL1K XL1K XL1K PLS2 XL1K PLS2 PLS2 PLS2 XL1K XL1K COMPLETION AUXILIARY AUXILIARY AUXILIARY NUCLEAR CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER IMAGING IMAGING CHLS PLS2 PLS2 PLS2 PLS2 XL1K PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 XL1K IMAGING XL1K IMAGING XL1K IMAGING IMAGING PLS2 PLS2
    • 293 Contractor Tool Mnemonics Mnemonic Tool CBL CBL CBL CBL CBL CBL CBL CBL_D CCAT CCATCL CCATGR CCL CCL CCL CCL CCL CBL CBLDC CBLEA CBLEB CBLFA CBLFB CBLHA FWAT-A CCAT M214 M507 CCLPA CCLPB CCLQA CCLRA CCLUN CCL CCL_1 CCL_PL CCLDC CCLDC1 CCLGP CCLHG CCLPET CCLPIB CCLSG CCLT CDL CCLWA M214 PCU COSMOS BELL GPL HGNC-A UCCL COSMOS SGNC-A CCLTAA CDLGA CDL CDL CDL CDLKA CDLKB CDLLA CDL CDL CDT CDTCAL CH_G CH_HOS CH_TEN CH2TEN CHARM CDLMA CDLNA CDT CDTCAL CHTN CHTN CHTN CHTN CHARM CHFW CHS CHSF FWAT-A FWAT-A CHSFAA CIC PENGO820 Description Type Code CEMENT BOND TOOL CEMENT BOND TOOL CEMENT BOND TOOL CEMENT BOND TOOL CEMENT BOND TOOL—(1 11/16 TOOL) CEMENT BOND TOOL—(1 11/16 TOOL) CEMENT BOND TOOL—(MODULAR) M305B DITS SHORT BOND MODE COMPENSATED CEMENT ATTEN TOOL M/C CASING COLLAR LOCATOR MULTICHANNEL GAMMA RAY CASING COLLAR LOCATOR—(PLT) CASING COLLAR LOCATOR—(PLT) CASING COLLAR LOCATOR—(DIGITAL) CASING COLLAR LOCATOR—(MODULAR) CASING COLLAR LOCATOR— (UNIVERSAL) CASING COLLAR LOCATOR—(DIGITAL) M/C CASING COLLAR LOCATOR CASING COLLAR LOCATOR DC CASING COLLAR LOCATOR DC CASING COLLAR LOCATOR GPL CASING COLLAR LOCATOR HGNC CASING COLLAR LOCATOR PET TOOL MAGNETIC CCL DC CASING COLLAR LOCATOR—PIB SGNC CASING COLLAR LOCATOR CASING COLLAR LOCATOR—(DITS) COMPENSATED DENSITY LOG TOOL— (3 3/8 TOOL) COMPENSATED DENSITY LOG TOOL COMPENSATED DENSITY LOG TOOL COMPENSATED DENSITY LOG TOOL— (2 3/4 TOOL) COMPENSATED DENSITY LOG TOOL COMPENSATED DENSITY LOG TOOL COMPENSATED DENSITY CDT CALIPER GO DOWNHOLE LOADCELL HOSTILE CABLE HEAD LOAD CELL DITS CABLE HEAD LOAD CELL D2TS CABLE HEAD LOAD CELL CASED HOLE ANALYSIS RESERVOIR MODEL M305 ACOUSTIC M305B CASED HOLE XTRA LS 4 RCVR DT CASED HOLE—SEQ. FORMATION TESTER—DDL CASING INSPECTION CALIPER CEMENT EVAL. CEMENT EVAL. CEMENT EVAL. CEMENT EVAL. CEMENT EVAL. CEMENT EVAL. CEMENT EVAL. CEMENT EVAL. CEMENT EVAL. COMPLETION NUCLEAR COMPLETION COMPLETION COMPLETION COMPLETION COMPLETION PLS2 XL1K XL1K XL1K XL1K XL1K XL1K PLS2 PLS2 PLS2 PLS2 XL1K XL1K XL1K XL1K XL1K COMPLETION COMPLETION COMPLETION COMPLETION COMPLETION COMPLETION COMPLETION COMPLETION COMPLETION COMPLETION COMPLETION NUCLEAR XL1K PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 XL1K XL1K NUCLEAR NUCLEAR NUCLEAR XL1K XL1K XL1K NUCLEAR NUCLEAR NUCLEAR CALIPER AUXILIARY AUXILIARY AUXILIARY AUXILIARY ANALYSIS XL1K XL1K PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 SONIC SONIC PRODUCTION PLS2 PLS2 XL1K CASING INSPECT. PLS2
    • 294 Well Logging and Formation Evaluation Mnemonic Tool Description CIT CIT CASING INSPECTION TOOL CIT CITAA CASING INSPECTION TOOL CIT_A CIT CASING INSPECTION (ACOUS COMB) CLAMS CNT CNT CNT CNT CNT CNT CNT CNT CNT CNT CNT CNT CNT CNT CNT_K CNT_N COM CORAL CORE CP_CAL CP_DEN CP_DN CP_GR CQPT CLAMS CNTAA CNTAB CNTBA CNTCA CNTDA CNTEA CNTFA CNTJA CNTKA CNTKB CNTLA CNTMA CNTNA CNTPA CNTKA CNTNA COM CORAL SWC COMPROBE COMPROBE COMPROBE COMPROBE CQPTAA CSNG CSNG CSNG CSNGGR CSNG CSNG CSNGMI CSNGTI CSNG_G CSNG_T CSNGMC D2TS DC_CAL DCHT DIEL DIELGR DIKA DIL DIL CSNGG-A CSNGT-A CSNG-MC D2TSAA DC CALIP DCHTAA DIEL DIEL DIKA DIL DILAA CLAY AND MATRIX ANALYSIS COMPENSATED NEUTRON—PAD STYLE COMPENSATED NEUTRON—PAD STYLE COMPENSATED NEUTRON—PAD STYLE COMPENSATED NEUTRON—PAD STYLE COMPENSATED NEUTRON—PAD STYLE COMPENSATED NEUTRON—PAD STYLE COMPENSATED NEUTRON—(2 3/4 TOOL) COMPENSATED NEUTRON—MANDREL COMPENSATED NEUTRON—MANDREL COMPENSATED NEUTRON—MANDREL COMPENSATED NEUTRON—MANDREL COMPENSATED NEUTRON—MANDREL COMPENSATED NEUTRON—MANDREL COMPENSATED NEUTRON—MANDREL COMPENSATED NEUTRON K MUX-E COMPENSATED NEUTRON MODEL N XL1—COMPUTED OUTPUTS PANEL COMPLEX RESERVOIR ANALYSIS MODEL M6 CORE GUN WITH SP COMPROBE CALIPER COMPROBE DENSITY COMPROBE DUAL NEUTRON COMPROBE GAMMA/CALIPER/CCL COMPENSATED QUARTZ PRESSURE TOOL COMPENSATED SPECTRAL NATURAL GAMMA COMPENSATED SPECTRAL NATURAL GAMMA MINI TRACERSCAN—SLIM HOLE COMPENSATED SPECTRAL NATURAL GAMMA DITS SPECTRAL GAMMA—GRAPHITE DITS SPECTRAL GAMMA—TITANIUM SPECTRAL GAMMA—GRAPHITE DITS 2 TELEMETRY SUB GO DC CALIPER DITS CABLEHEAD TENSION SUB DIELECTRIC TOOL DIELECTRIC TOOL GAMMA RAY LATEROLOG 3 DIK-A DUAL INDUCTION DUAL INDUCTION Type Code CASING INSPECT.PLS2 CASING INSPECT. CASING INSPECT.PLS2 ANALYSIS NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR ANALYSIS ANALYSIS SAMPLING CALIPER NUCLEAR NUCLEAR NUCLEAR PRODUCTION PLS2 PLS2 XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K PLS2 PLS2 XL1K PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 XL1K NUCLEAR CHLS NUCLEAR XL1K NUCLEAR NUCLEAR XL1K XL1K NUCLEAR NUCLEAR NUCLEAR TELEMETRY CALIPER AUXILIARY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY PLS2 PLS2 PLS2 XL1K PLS2 XL1K PLS2 PLS2 PLS2 PLS2 XL1K XL1K PLS2
    • 295 Contractor Tool Mnemonics Mnemonic Tool DIL DIL DIL DIL DIL DIL DIL DIL DIL DIL DIL DIL DILT DIND DIOHD DIP_MC DIPCOR DILBA DILBB DILBC DILBD DILCA DILCB DILCC DILDA DILEA DILFA DILGA DILHA DILTAA DILT-A B CHTN M242-A DIPCOR DITCCL DITSHD DLD DLL DLL DLL DLL DLL DLL DLL DLL DLL DLL DLL DLLT DLLT DLLT DLLX DMSFL DMSFLC DMSFLX DSEN DSEN DSN_II DSNT DSTU DSTU DTD DTD DTEN EMI2 DCCL-A DUMMY DLDAA AA-EB DLLAA DLLBA DLLBB DLLCA DLLDA DLLEA DLLEB DLLFA DLLGA DLLHA DLLT-A DLLTAA DLLTBA FA,GA,HA DMSFL MSFL MSFL DSEN-A DSENAA DSNT-A DSNTAA DSTUAA DSTUBA DTDAA DTDBA CHTN EMI-B Description Type Code DUAL INDUCTION DUAL INDUCTION DUAL INDUCTION DUAL INDUCTION DUAL INDUCTION DUAL INDUCTION DUAL INDUCTION DUAL INDUCTION DUAL INDUCTION DUAL INDUCTION DUAL INDUCTION DUAL INDUCTION DITS DUAL INDUCTION—(W/MGRD) DITS DUAL INDUCTION SVC HEADER/ SIG COND/ DIO STAT M/C DIPMETER INSTRUMENT FOUR ARM DIPMETER ANALYSIS (for MC DIP tools) DITS CCL DITS SERVICE HEADER + ANALOG DDL DLL LOWER ELECTRODE DDL STANDARD DUAL LATEROLOG DDL STANDARD DUAL LATEROLOG DDL STANDARD DUAL LATEROLOG DDL STANDARD DUAL LATEROLOG DDL STANDARD DUAL LATEROLOG DDL STANDARD DUAL LATEROLOG DDL STANDARD DUAL LATEROLOG DDL STANDARD DUAL LATEROLOG DDL EXTENDED DUAL LATEROLOG DDL EXTENDED DUAL LATEROLOG DDL EXTENDED DUAL LATEROLOG DITS DUAL LATEROLOG DITS DUAL LATEROLOG DITS DUAL LATEROLOG DDL EXTENDED DUAL LATEROLOG STANDALONE DITS MSFL DITS STANDALONE MSFL / MICLOG DITS STANDALONE MSFL (EXP) DUAL SPACED EPITHERMAL NEUTRON DUAL SPACED EPITHERMAL NEUTRON DUAL SPACED NEUTRON II DUAL SPACED NEUTRON II DITS SUBSURFACE TELEMETRY UNIT DITS SUBSURFACE TELEMETRY UNIT DOWNHOLE LINE TENSION DOWNHOLE LINE TENSION DIFFERENTIAL TENSION DITS ELECTRIC MICRO IMAGER— (SINGLE DITS) RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY DUMMY TOOL DIPMETER ANALYSIS XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K PLS2 PLS2 PLS2 PLS2 COMPLETION DUMMY TOOL RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY NUCLEAR NUCLEAR NUCLEAR NUCLEAR TELEMETRY TELEMETRY AUXILIARY AUXILIARY AUXILIARY IMAGING PLS2 PLS2 XL1K PLS2 XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K PLS2 XL1K XL1K PLS2 PLS2 PLS2 PLS2 PLS2 XL1K PLS2 XL1K XL1K XL1K XL1K XL1K PLS2 PLS2
    • 296 Mnemonic Well Logging and Formation Evaluation Tool EMI2CL EMI2DP EMI2MG EMID2 EMI-A EMI-A EMI-A EMI-B EMIRES EVR_DN EVR_GR EVRSD8 EVRSDL FACMAN FACT FDF FDT FDT FDT FED FED FED FED FEDNAV FHY FIAC FIAC FLD_PL FLDN FLTT FMS FMS FMS FWL2DT FWS FWS_D2 EMI-B DSNT-A NGRT-A SDLT-A SDLT-A FACT-A FACTAA FDFAA FDTEA FDTEB FDTEC FED FEDGA FEDHA FEDJA FED FHYGA FIAC-A FIACAA PCU BELL M139-A FMSHA FMSHB FMSHC FWAT-A FWAT-A FWAT-A FWST FWST2 FWST23 FWST2A FWST2S FWST2U FWST4 FWST4A FWSTA FWSTA8 FWSTAU FWSTD2 FWSTDA G-CBL G_GR G_SFT4 FWSTAA FWAT-A FWAT-A FWAT-A FWAT-A FWAT-A FWAT-A FWAT-A FWAT-A FWAT-A FWAT-A FWAT-A FWAT-A CBLEB GO GR G_SFT4 Description EMI CALIPER PACKAGE EMI DIPMETER PACKAGE EMI NAVIGATION MAGNETOMETER DITS ELECTRIC MICRO IMAGER— (DOUBLE DITS) EMI RESISTIVITY PACKAGE DUAL SPACED NEUTRON II (EVR-II) GAMMA RAY TOOL (EVR-II) SPECTRAL DENSITY (8 BIT) (EVR-II) SPECTRAL DENSITY (EVR-II) FACT CALIPER MANDREL FOUR ARM CALIPER TOOL—DITS FLOW DIVERTER FLOWMETER FLUID DENSITY—MUXB PL FLUID DENSITY—MUXB PL FLUID DENSITY—MUXB PL DIO FOUR ELECTRODE DIPMETER FOUR ELECTRODE DIPMETER (4 1/2INCH) FOUR ELECTRODE DIPMETER (3 1/2 IN) FOUR ELECTRODE DIPMETER (4 1/2 IN) FED NAVIGATION PACKAGE (G) FULL BORE HYDRO TOOL FOUR INDEPENDENT ARM CALIPER FOUR INDEPENDENT ARM CALIPER FLUID DENSITY DC FLUID DENSITY FLUID TRAVEL TOOL HI RES FLOWMETER SPINNER HI RES FLOWMETER SPINNER HI RES FLOWMETER SPINNER M305A 2 RCVR LONG SPACED—SNR-DT M305B 4 RCVR AUTO GAIN M305B 4 RCVR AUTO GAIN—QUAD STACK M305A FULL WAVE SONIC M305A ACOUSTIC M305A THREE RECEIVER LONG SPACE M305A ACOUSITIC/AMPL M305A TWO RECEIVER LONG SPACED M305A 2 RCVR LONG SPACED—DT M305A ACOUSTIC SHORT TIP MODE 4 M305A ACOUSTIC SHORT BOND MODE M305A ACOUSTIC M305A ACOUSTIC M305A ACOUSTIC (MODE A) FOR INT’L M305A ACOUSTIC DITS-2 M305A ACOUSTIC DITS-2 CEMENT BOND TOOL DDL GAMMA RAY TOOL G SERIES SFT4 Type Code CALIPER RESISTIVITY AUXILIARY IMAGING PLS2 PLS2 PLS2 PLS2 RESISTIVITY NUCLEAR NUCLEAR NUCLEAR NUCLEAR CALIPER CALIPER PRODUCTION PRODUCTION PRODUCTION PRODUCTION DIPMETER DIPMETER DIPMETER DIPMETER DIPMETER PRODUCTION CALIPER CALIPER PRODUCTION PRODUCTION PRODUCTION PRODUCTION PRODUCTION PRODUCTION SONIC SONIC SONIC PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 XL1K XL1K XL1K XL1K XL1K PLS2 XL1K XL1K XL1K PLS2 XL1K PLS2 XL1K PLS2 PLS2 PLS2 XL1K XL1K XL1K PLS2 PLS2 PLS2 SONIC SONIC SONIC SONIC SONIC SONIC SONIC SONIC SONIC SONIC SONIC SONIC SONIC CEMENT EVAL. NUCLEAR SAMPLING XL1K PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 CHLS PLS2 PLS2
    • 297 Contractor Tool Mnemonics Mnemonic Tool G_TEMP GAMMA GR GR GR GR GR GR GR GR_DC GR_DC1 GR_DC2 GR_DN GR_DSN GR_GP GR_HG GR_MC1 GR_MC2 GR_PIB GR_PL GR_SG GRAD_P GRAVEL GRPERF HDIL HDIL HDSN HDSN HECT HECT HEDNAV HEDT HETS HFDT HFDT HFDTAN HFWS HFWS2 HFWS28 HFWS2A TMP-IC NGRT-A GRDC GRGA GRHA GRIA GRLA GRRA GRRB COSMOS BELL COSMOS M507 M507 GPL HGNC-A M507 M507 COSMOS MC SGNC-A SONDEX GRAVEL M157 HDIL-A HDILAA HDSN-A HDSNAA HECT-A HECTAA HEDT HEDT-A HETSAA HFDT HFDTAA HFDT HFWSAA HFWS-A HFWS-A HFWS-A HFWS4 HFWS4A HFWSA HFWSA8 HGNI HMST HMSTQ HNGR HNGR HFWS-A HFWS4 HFWS-A HFWS-A HNGIAA HMST-A HMSTQ HNGR-A HNGRAA Description G_SERIES TEMPERATURE TOOL GAMMA RAY TOOL NATURAL GAMMA NATURAL GAMMA—SLIM HOLE NATURAL GAMMA NATURAL GAMMA NATURAL GAMMA NATURAL GAMMA—MUX PL NATURAL GAMMA—MUX PL DC NATURAL GAMMA DC NATURAL GAMMA DC NATURAL GAMMA MULTICHANNEL GAMMA RAY MULTICHANNEL GAMMA RAY GPL NATURAL GAMMA HGNC NATURAL GAMMA MULTICHANNEL GAMMA RAY MULTICHANNEL GAMMA RAY DC NATURAL GAMMA—PIB NATURAL GAMMA SGNC NATURAL GAMMA M/C SONDEX GRADIOMANOMETER GRAVEL PACK ANALYSIS M157 GAMMA PERFORATOR HOSTILE DUAL INDUCTION HOSTILE DUAL INDUCTION HOSTILE DUAL SPACED NEUTRON HOSTILE DUAL SPACED NEUTRON HOSTILE FOUR ARM CALIPER HOSTILE FOUR ARM CALIPER HEDT NAVIGATION PACKAGE(G) HOSTILE ENV. DIPMETER TOOL HOSTILE TELEMETRY SUB—DITS DITS HIGH FREQUENCY DIELECTRIC DITS HIGH FREQUENCY DIELECTRIC DITS HIGH FREQUENCY DIELECTRIC HOSTILE FULL WAVE SONIC HOSTILE SONIC—LONG SPACE 2TR HOSTILE SONIC—LONG SPACE 2TR HOSTILE SONIC—LONG SPACE 2TR AMPLITUDE HOSTILE SONIC—SHORT SPACE HEST ACOUSTIC SHORT BOND MODE HOSTILE SONIC—FULL WAVE (A) HOSTILE SONIC—FULL WAVE (A) HOSTILE CCL / GAMMA / NEUTRON INST. HMST FORMATION TESTER HYBRID MST—QUARTZ TRANSDUCER HOSTILE GAMMA DETECTOR HOSTILE GAMMA DETECTOR Type Code PRODUCTION NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR PRODUCTION ANALYSIS PRODUCTION RESISTIVITY RESISTIVITY NUCLEAR NUCLEAR CALIPER CALIPER DIPMETER DIPMETER TELEMETRY RESISTIVITY RESISTIVITY RESISTIVITY SONIC SONIC SONIC SONIC PLS2 PLS2 XL1K XL1K XL1K XL1K XL1K XL1K XL1K PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 XL1K PLS2 XL1K PLS2 XL1K PLS2 PLS2 XL1K PLS2 XL1K PLS2 XL1K PLS2 PLS2 PLS2 SONIC SONIC SONIC SONIC AUXILIARY SAMPLING SAMPLING NUCLEAR NUCLEAR PLS2 PLS2 PLS2 PLS2 XL1K PLS2 PLS2 PLS2 XL1K
    • 298 Well Logging and Formation Evaluation Mnemonic Tool HPDC HPDC_D HPDC_N HPDL HPDCAA HPDCD-A HPDCN-A HPDLAA HPDU HPDUAA HRI HRI HRI HRTT HRTTB HRTTI HSDI HSDL_I HSDL_M HSDM HSDP HSN HYD HYD HYD IEL IEL IEL IEL IEL IEL IEL HRI HRIBA HRICA HRTTAA HRTT-A HRTT-A HSDIAA HSDLI-A HSDLM-A HSDMAA HSDPAA HSN-A HYDFA HYDFB HYDFC IELAA IELBA IELCA IELDA IELDB IELEA IELFA IEL LFD LFD_D2 IELGA LFD LFD LFD2DT LFD2MT LFDDT LFDMT LFDT LFDT LFDT8 LFDTDT LFDTM LFDTMT LFS LFD LFD LFD LFD LFDT LFDTAA LFDT LFDT LFDT LFDT LFD LFS_D2 LFD LFS_Q2 LFD Description Type Code HSDL—DENSITY DECENTRALIZER DENSITY DECENTRALIZER NEUTRON DECENTRALIZER HSDL—POWERED DECENTRALIZER— LOWER HSDL—POWERED DECENTRALIZER— UPPER HIGH RESOLUTION INDUCTION HIGH RESOLUTION INDUCTION HIGH RESOLUTION INDUCTION DITS TEMPERATURE TOOL—INLINE DITS TEMPERATURE TOOL—BOTTOM DITS TEMPERATURE TOOL—INLINE HOSTILE SPECTRAL DENSITY INST HOSTILE SDL INLINE HOSTILE SDL MANDREL HOSTILE SPECTRAL DENSITY MANDREL HOSTILE SPECTRAL DENSITY PAD HOSTILE SHORT NORMAL HYDRO TOOL CENTER SAMPLE HYDRO TOOL CENTER SAMPLE HYDRO TOOL CENTER SAMPLE INDUCTION ELECTRIC TOOL INDUCTION ELECTRIC TOOL INDUCTION ELECTRIC TOOL INDUCTION ELECTRIC TOOL INDUCTION ELECTRIC TOOL INDUCTION ELECTRIC TOOL INDUCTION ELECTRIC TOOL— (2 3/4 INCH TOOL) INDUCTION ELECTRIC TOOL M305B MONO/DIPOLE—FILTER SELECT M305B LF MONO P TOOL; DIPOLE XMTR—HIGH DATA RATE M305B LF DIPOLE TOOL; DIPOLE XMTR M305B LF DIPOLE TOOL; MONO P XMTR M305B LF DIPOLE TOOL; DIPOLE XMTR M305B LF DIPOLE TOOL; MONO P XMTR LOW FREQ DIPOLE ACOUSTIC LOW FREQ DIPOLE ACOUSTIC LOW FREQ DIPOLE ACOUSTIC LF DIPOLE TOOL; DIPOLE XMTR. LFDT ACOUSTIC; MONOPOLE XMTR. LF DIPOLE TOOL; MONO P XMTR. M305B LSS / FWS CONCURRENT 6≤ RES @ 34 FPM M305B LSS / FWS CONCURRENT 3≤ RES @ 34 FPM (HIGH DATA RATE) M305B LSS / FWS CONCURRENT 6≤ RES @ 34 FPM—QUAD STACK CENTRALIZER CALIPER CALIPER NUCLEAR XL1K PLS2 PLS2 XL1K NUCLEAR XL1K RESISTIVITY RESISTIVITY RESISTIVITY TEMPERATURE TEMPERATURE TEMPERATURE AUXILIARY NUCLEAR NUCLEAR NUCLEAR NUCLEAR RESISTIVITY PRODUCTION PRODUCTION PRODUCTION RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY PLS2 XL1K XL1K XL1K PLS2 PLS2 XL1K PLS2 PLS2 XL1K XL1K PLS2 XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K RESISTIVITY SONIC SONIC XL1K PLS2 PLS2 SONIC SONIC SONIC SONIC SONIC SONIC SONIC SONIC SONIC SONIC SONIC PLS2 PLS2 PLS2 PLS2 PLS2 XL1K PLS2 PLS2 PLS2 PLS2 PLS2 SONIC PLS2 SONIC PLS2
    • 299 Contractor Tool Mnemonics Mnemonic Tool LIDA LL3 LSS LIDA LL3 FWAT-A LSS LSS LSS LSS LSS_D2 LSSEA LSSIA LSSKA LSSLA FWAT-A LSS_FF FWAT-A LSSAFW LSSFW M/CHD M_FLOW M_GRAD M_LOG1 M202 M202 M305 M305 DUMMY PCU PCU HFDT M202 M202AA M213 M214 M271 M271_C M271_O M271D M271D2 M307 M307A M307D M307V M310 M331 M333 M334 M507 M507M M904C MAC MAC MAC MAC MAC MAC MAC MAC MAC MAC MAC M213 M214 M271 M271 M271 M271 M271 M307 M307 M307D M307 M310 M331 M507 M334 M507 M507M 904-C MAC MACAA MACAB MACAC MACBA MACBB MACBC MACBD MACBE MACCA MACCB Description LITHOLOGY IDENTIFICATION ANALYSIS LATEROLOG 3 M305B ALL RCVRS FIXED GAIN—DT ONLY LONG SPACED SONIC—(WVF) LONG SPACED SONIC—(WVF) LONG SPACED SONIC—(WVF) LONG SPACED SONIC—(WVF) M305B 4 RCVRS FIXED GAIN—QUAD STACK M305B 3 RCVRS FIXED GAIN—FRAC FINDER M305 Long Spaced Sonic—Mode M305 COMBINATION Sonic—Mode M/C SERVICE HEADER + ANALOG M/C SONDEX FULLBORE FLOWMETER M/C SONDEX GRADIOMANOMETER HFDT MICROLOG MULTICHANNEL CALIPER CALIPER—TWO ARM—DITS BOW SPRING MULTICHANNEL CALIPER M/C CASING COLLAR LOCATOR MULTICHANNEL BOND TOOL MULTICHANNEL SONIC CASED HOLE MULTICHANNEL SONIC OPEN HOLE M/C M271 DIGITAL M271 DIGITAL—4 RCVR TO TAPE CEMENT BOND TOOL (SINGLE CHAN) SINGLE CHANNEL SONIC AMPLITUDE DIGITAL SINGLE CHANNEL SONIC SINGLE CHANNEL SONIC VELOCITY MULTICHANNEL DUAL INDUCTION M/C DUAL SPACED NEUTRON MULTICHANNEL GAMMA RAY MULTICHANNEL GAMMA RAY MULTICHANNEL GAMMA RAY MULTICHANNEL GAMMA RAY MULTI-CHANNEL DUAL GUARD MC MULTI-ARM CALIPER TOOL MULTI-ARM CALIPER TOOL MULTI-ARM CALIPER TOOL MULTI-ARM CALIPER TOOL MULTI-ARM CALIPER TOOL MULTI-ARM CALIPER TOOL MULTI-ARM CALIPER TOOL MULTI-ARM CALIPER TOOL MULTI-ARM CALIPER TOOL MULTI-ARM CALIPER TOOL MULTI-ARM CALIPER TOOL Type Code ANALYSIS RESISTIVITY SONIC PLS2 PLS2 PLS2 SONIC SONIC SONIC SONIC SONIC XL1K XL1K XL1K XL1K PLS2 SONIC PLS2 SONIC SONIC DUMMY TOOL PRODUCTION PRODUCTION RESISTIVITY CALIPER CALIPER PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 XL1K CALIPER PRODUCTION CEMENT EVAL. CEMENT EVAL. SONIC CEMENT EVAL. CEMENT EVAL. CEMENT EVAL. CEMENT EVAL. CEMENT EVAL. SONIC RESISTIVITY NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR RESISTIVITY CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER PLS2 PLS2 CHLS PLS2 PLS2 PLS2 PLS2 CHLS PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K
    • 300 Mnemonic Well Logging and Formation Evaluation Tool MAC MAC MAC MAC MAC MAC MAC MAC_DC MACTDC MC MACCC MACDA MACDB MACDC MACEA MACEB MACEC MAC-AA MAC-AA MC MC MCAA MC_CCL MC_DSN MC_GN MC_PL MCCEB MCCL MCDSN MCFRXO MCGRD MDIP MEL MEL MEL MEL MEL_DC MELCAL MELPUL MGCAL MGRD MICLGC MICLOG MLL MLL MLL MSFCAL MSFL MSFL MSFL MSFL MSFL MSFL MSFL MSFL40 MSFLM MSFT MSFT MSFT M214 M265A DUMMY DUMMY DUMMY M214 M265A MGRD M320 M243A MELCA MELDA MELDB MELDC CA MEL DA,DB,DC MGRD MGRDAA MSFL SDLT-A MLLAA MLLBA MLLCA MSFL MSFL MSFLCA MSFLDA MSFLDB MSFLDC MSFLEA MSFLFA MSFL40 M320 MSFTAA MSFTLM MSFTUM Description MULTI-ARM CALIPER TOOL MULTI-ARM CALIPER TOOL MULTI-ARM CALIPER TOOL MULTI-ARM CALIPER TOOL MULTI-ARM CALIPER TOOL MULTI-ARM CALIPER TOOL MULTI-ARM CALIPER TOOL MULTI-ARM CALIPER TOOL MULTI-ARM CALIPER TOOL MULTICHANNEL PPM SUB / CIT / DSN / GEN PURPOSE MULTICHANNEL PPM SUB (DUMMY TOOL) M/C CASING COLLAR LOCATOR M/C DUAL SPACED NEUTRON DUMMY MC PRODUCTION MULTICHANNEL SUB MC CABLE ELECTRODE BRIDLE M/C CASING COLLAR LOCATOR M/C DUAL SPACED NEUTRON M265A MULTI_CHANNEL MICRO GUARD MICRO GUARD-DLLT-A MULTICHANNEL DIPMETER MANDREL MICRO ELECTRIC LOG—DC MICRO ELECTRIC LOG—DC MICRO ELECTRIC LOG—DC MICRO ELECTRIC LOG—DC MICRO ELECTRIC LOG—DC DDL MEL CALIPER MICRO ELECTRIC LOG—DDL—PULSE MULTI-CHANNEL MICRO GUARD CAL MICRO GUARD RESISTIVITY-DLLT-A MICROLOG ON DITS MSFL MICROLOG—SDLT MICROLATEROLOG RESISTIVITY MICROLATEROLOG RESISTIVITY MICROLATEROLOG RESISTIVITY MSFL CALIPER DDL MICRO-SPHERICALLY FOCUSED MICRO-SPHERICALLY FOCUSED-DDL MICRO-SPHERICALLY FOCUSED-DDL MICRO-SPHERICALLY FOCUSED-DDL MICRO-SPHERICALLY FOCUSED-DDL MICRO-SPHERICALLY FOCUSED-DDL MICRO-SPHERICALLY FOCUSED-DDL DDL MSFL LOW RES OPTION X40 MSFL MANDREL WITH DITS DLL MICRO-SPHERICALLY FOCUSED-DITS MICRO-SPHERICALLY FOCUSED-DITS MICRO-SPHERICALLY FOCUSED-DITS Type Code CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER COMPLETION XL1K XL1K XL1K XL1K XL1K XL1K XL1K PLS2 PLS2 CHLS DUMMY TOOL XL1K COMPLETION NUCLEAR DUMMY TOOL TELEMETRY AUXILIARY COMPLETION NUCLEAR RESISTIVITY RESISTIVITY DIPMETER RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY CALIPER RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY CALIPER RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY RESISTIVITY PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 XL1K XL1K XL1K XL1K PLS2 PLS2 PLS2 PLS2 XL1K PLS2 PLS2 XL1K XL1K XL1K PLS2 PLS2 XL1K XL1K XL1K XL1K XL1K XL1K PLS2 PLS2 XL1K XL1K XL1K
    • 301 Contractor Tool Mnemonics Mnemonic Tool MTRACM MC-CSNG MUX MUXLA Description MUXPL NAVS NE_HG NE_SG NEU MUXPL NAVS-A HGNC-A SGNC-A NEUAA NEU NEUBA NEU NEUCA NEUDC NEUDC1 NEUPIB NGRT PCK PCUA COSMOS COSMOS COSMOS NGRTAA PCK PCUA PCUB PCUB PET PET PET PET PET/CB PET_C PET PET PETAA PETBB PET/CB PET_C PETGR PGPP UGR PIT PIT MC CSNG MINI TOOL MUX TELE FOR NEUTRON—SUPER STACK MUXB-PL DATA TOOL FACT NAVIGATION SUB NEUTRON HE3 DETECTOR NEUTRON HE3 DETECTOR CASE HOLE NEUTRON (1 IN.)—SINGLE DETECTOR CASE HOLE NEUTRON (1.68 IN.)—SINGLE DETECTOR CASE HOLE NEUTRON (3.5 IN.)—SINGLE DETECTOR DC NEUTRON (15≤ SOURCE SUB) DC NEUTRON (15≤ SOURCE SUB) DC NEUTRON (15≤ SOURCE SUB)—(PIB) GAMMA RAY TOOL-DITS PANEL—XL1-GRAVEL PACK MULTICHANNEL PRODUCTION LOGGING TOOL MULTICHANNEL PRODUCTION LOGGING TOOL PULSE ECHO TOOL (MUX-B) PULSE ECHO TOOL (MUX-B) PULSE ECHO TOOL (MUX-B) PULSE ECHO TOOL (MUX-B) PULSE ECHO TOOL (MUX-B) PULSE ECHO TOOL (MUX-B)—(QC JOINTS) PET TOOL GAMMA RAY PANEL—GENERAL PURPOSE DDL HEADER DITS PIPE INSPECTION TOOL PIT8 PIT DITS PIPE INSPECTION TOOL-8 PAD PL_HD PLA PQ_PL PR_PL PSGT PSGT PSGTMD PSYS QPG QPG ROTASC MC PLA PCU PCU PSGT-A PSGTAA ANAL SASHA SDDT SASHA NAV-DDT PROD. LOGGING HEADER + ANALOG PRODUCTION LOG ANALYSIS PRESSURE-(PETRO-QUARTZ) PRESSURE—(WELL TEST) PULSE SPECTRAL GAMMA TOOL PULSE SPECTRAL GAMMA TOOL PSGT TMD PROCESSING PANEL—DDL SYSTEM GENERAL STATUS QUARTZ PRESSURE GUAGE QUARTZ PRESSURE GUAGE ROTASCAN (MUST BE RUN WITH TSCAN 1 11/16) SANDY SHALE ANALYSIS MODEL STANDALONE DITS DIRECTIONAL QPGAA QPGBA ROTASC Type Code NUCLEAR TELEMETRY PLS2 XL1K PRODUCTION AUXILIARY NUCLEAR NUCLEAR NUCLEAR CHLS PLS2 PLS2 PLS2 XL1K NUCLEAR XL1K NUCLEAR XL1K AUXILIARY AUXILIARY AUXILIARY NUCLEAR DUMMY TOOL PRODUCTION PLS2 PLS2 PLS2 XL1K XL1K CHLS PRODUCTION CHLS CEMENT EVAL. CEMENT EVAL. CEMENT EVAL. CEMENT EVAL. CEMENT EVAL. CEMENT EVAL. CHLS PLS2 XL1K XL1K CHLS PLS2 NUCLEAR DUMMY TOOL PLS2 XL1K CASING INSPECT. CASING INSPECT. DUMMY TOOL ANALYSIS COMPLETION COMPLETION NUCLEAR NUCLEAR ANALYSIS DUMMY TOOL PRODUCTION PRODUCTION NUCLEAR PLS2 PLS2 PLS2 PLS2 PLS2 PLS2 XL1K PLS2 XL1K XL1K XL1K CHLS ANALYSIS AUXILIARY PLS2 PLS2 PLS2
    • 302 Mnemonic Well Logging and Formation Evaluation Tool SDDT SDL SDL8 SDLT SDLT SDLT SED SED SED SDDTA SDLT-A SDLT-A B SDLTAA SDLTBA SDLTBB SED-C SEDAO SEDAW SED SEDBO SED SEDBW SED SEDS SEDNAV SEDRES SFT SED-C SED-RES SFTDA SFT SFTDB SFT SFTDC SFT SFTEA SFT SFTEB SFT SFTFA SFT SFTGA SFT SFTGB SFT SFTHA SFT SFTJA SFT SFTLA SFT SFT4 SFTC SFTI SFTT-A SFT4 SFTC SFTIAA SFTPQ SFTT SGR SGR SGR SFTT-B SFTTAA SGR SGRAC SGRBA Description STANDALONE DITS DIRECTIONAL SPECTRAL DENSITY LOG SPECTRAL DENSITY LOG (8 BIT) SPECTRAL DENSITY LOG—(8 BIT) SPECTRAL DENSITY LOG—(12 BIT) SPECTRAL DENSITY—(FLOATING BODY) SIX ELECTRODE DIPMETER—DITS SIX ELECTRODE DIPMETER—(OIL BASE) SIX ELECTRODE DIPMETER—(WATER BASE) SIX ELECTRODE DIPMETER—(OIL BASE) DITS SIX ELECTRODE DIPMETER—(WATER BASE) DITS SIX ELECTRODE DIPMETER—(SONIC COMBO) DITS SED NAVIGATION PACKAGE (G) SED HI-RESOLUTION RESISTIVITY SEQ. FORMATION TESTER—DDL—TYPE III SEQ. FORMATION TESTER—DDL—TYPE III SEQ. FORMATION TESTER—DDL—TYPE III SEQ. FORMATION TESTER—DDL—TYPE III SEQ. FORMATION TESTER—DDL—TYPE III SEQ. FORMATION TESTER—DDL—SLIM HOLE SEQ. FORMATION TESTER—DDL—TYPE IV SEQ. FORMATION TESTER—DDL—TYPE IV SEQ. FORMATION TESTER—DDL—TYPE IV SEQ. FORMATION TESTER—DDL—TYPE IV SEQ. FORMATION TESTER—DDL—TYPE IV SEQUENTIAL FORMATION TESTER SFT4 PETRO QUARTZ SFT PETRO QUARTZ TOOL (SFTT-C) SEQ. FORMATION TESTER—DITS INSTRUMENT SFT PETRO QUARTZ TOOL SEQ. FORMATION TESTER—DITS SPECTRAL GAMMA RAY (DIO #9) SPECTRAL GAMMA RAY TOOL SPECTRAL GAMMA RAY TOOL Type Code AUXILIARY NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR DIPMETER DIPMETER DIPMETER XL1K PLS2 PLS2 XL1K XL1K XL1K PLS2 XL1K XL1K DIPMETER XL1K DIPMETER XL1K DIPMETER XL1K AUXILIARY DIPMETER SAMPLING PLS2 PLS2 XL1K SAMPLING XL1K SAMPLING XL1K SAMPLING XL1K SAMPLING XL1K SAMPLING XL1K SAMPLING XL1K SAMPLING XL1K SAMPLING XL1K SAMPLING XL1K SAMPLING XL1K SAMPLING SAMPLING SAMPLING SAMPLING PLS2 PLS2 PLS2 XL1K SAMPLING SAMPLING NUCLEAR NUCLEAR NUCLEAR PLS2 XL1K PLS2 XL1K XL1K
    • 303 Contractor Tool Mnemonics Mnemonic Tool SGRD SGRD SHIVA SHIVA4 SHVOMN DSGT-A B SGRDAA SHIVA SHIVA4 SHVOMN SILT SLD SLD SLD SLD SILT SLD-A SLDBA SLDBB SLDDA SLD_BB SLDCAL SNP SNP SNP SNP SNPCAL SOTX SP_HRI SPC SPIN SPL SPN_PL SPT SPT SPT SSNT SSS SLD-A SLDCAL SNP SNPAA SNPBA SNPBB SNPCAL CALIPER HRI SPCAA BELL SPLGB PL SPTCA SPTCB SPTCC SILT FWAT-A STOP SWN TAC TAC TEMP TEMPSW TIP TMD TMD TMD TMDGR TMDL STOP M166 TACBA TACCA BELL SWN TIPAA M395 M395 TMDWX M395 TMDL TMDL TMDLAA TMP TMP TMP TMP_PL TMPIA TMPIB TMPIC PCU Description SHORT GUARD RESISTIVITY SUB—(DILT) SHORT GUARD RESISTIVITY SUB—(DILT) SIX ELECTRODE DIPMETER ANALYSIS FOUR ELECTRODE DIPMETER ANALYSIS SIX ELECTRODE DIPMETER OMNIPLOT ANALYSIS SLIM LINE INDUCTION (2 3/4≤) SPECTRAL LITHO-DENSITY—(DIO #7) SPECTRAL LITHO-DENSITY TOOL SPECTRAL LITHO-DENSITY TOOL SPECTRAL LITHO-DENSITY TOOL— (MUX-B) SPECTRAL LITHO-DENSITY—(DIO #7) SLD CALIPER SIDEWALL NEUTRON SIDEWALL NEUTRON SIDEWALL NEUTRON SIDEWALL NEUTRON SNP CALIPER DSTU STANDOFF TOOL—(CALIPER) ANALOG SP—MODIFIED HRI TOOL PRODUCTION LOGGING CENTRALIZER DC SPINNER PRODUCTION LOGGING MUX-B SUB SPINNER PAINE PRESSURE TOOL PAINE PRESSURE TOOL PAINE PRESSURE TOOL SLIM LINE SHORT NORM (2 3/4≤) M305B 2 RCVR FIXED GAIN SHORT SPACE TIP STOP CHECKS—PL PANEL SIDEWALL NEUTRON TWO ARM CALIPER—DDL TWO ARM CALIPER—DDL DC TEMPERATURE SWN TEMPERATURE DDL SUPER STACK TOP ISOLATION SUB THERMAL MULTI-GATE DECAY (TMD) THERMAL MULTI-GATE DECAY (TMD) THERMAL MULTI-GATE DECAY (TMD) TMD GAMMA RAY THERMAL MULTI-GATE DECAY LITHOLOGY TOOL THERMAL MULTI-GATE DECAY LITHOLOGY TOOL TEMPERATURE PANEL—GENERAL TEMPERATURE PANEL—GENERAL TEMPERATURE PANEL—GENERAL TEMPERATURE—(MC) Type Code RESISTIVITY RESISTIVITY ANALYSIS ANALYSIS ANALYSIS PLS2 XL1K PLS2 PLS2 PLS2 RESISTIVITY NUCLEAR NUCLEAR NUCLEAR NUCLEAR PLS2 PLS2 XL1K XL1K XL1K NUCLEAR CALIPER NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR AUXILIARY AUXILIARY CENTRALIZER PRODUCTION PRODUCTION PRODUCTION PRODUCTION PRODUCTION PRODUCTION RESISTIVITY SONIC PLS2 PLS2 PLS2 XL1K XL1K XL1K PLS2 PLS2 PLS2 XL1K PLS2 XL1K PLS2 XL1K XL1K XL1K PLS2 PLS2 PRODUCTION NUCLEAR CALIPER CALIPER TEMPERATURE TEMPERATURE AUXILIARY NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR XL1K PLS2 XL1K XL1K PLS2 PLS2 XL1K CHLS PLS2 XL1K PLS2 CHLS NUCLEAR XL1K COMPLETION COMPLETION COMPLETION TEMPERATURE XL1K XL1K XL1K PLS2
    • 304 Mnemonic Well Logging and Formation Evaluation Tool TPH_PL TPL TPL TRACER TRACER TRACMC TTRACE TTRACM TTTC TVS PCU TPLAA TPLBB CSNGG-A TRACER MC-CSNG CSNGT-A MC-CSNG TTTCAA TVSAA TVS TVSBA V_REG VCR VRS VRS WVF XYC XYC XYC XYC XYC XYC V_REG VCR VRSAA VRSCA WVFAA XYCAA XYCBA XYCCA XYCDA XYCDB XYCXX Description Type Code TEMPERATURE HI RES TOOL PUSHER PANEL—MUX A/B TOOL PUSHER PANEL—MUX A/B DITS TRACER SCAN (GRAPHITE) MC TRACERSCAN—MINITOOL MC CSNG LARGE TOOL—GRAPHITE DITS TRACER SCAN (TITANIUM) MC CSNG LARGE TOOL—TITANIUM THRU TUBING TELEMETRY CARTRIDGE CIRCUMFERENCIAL ACOUSTIC SCANNING TOOL CIRCUMFERENCIAL ACOUSTIC SCANNING TOOL VOLTAGE REGULATOR SUB VCR—TOOL SIMULATOR MODE VOLTAGE REGULATOR SUB VOLTAGE REGULATOR SUB WAVEFORM PANEL-(LSS)—DDL GENERAL X-Y CALIPER TOOL X-Y CALIPER TOOL X-Y CALIPER TOOL X-Y CALIPER TOOL X-Y CALIPER TOOL X-Y CALIPER TOOL—(CALIF. OPTION) TEMPERATURE AUXILIARY AUXILIARY NUCLEAR NUCLEAR NUCLEAR NUCLEAR NUCLEAR TELEMETRY IMAGING PLS2 XL1K XL1K PLS2 CHLS PLS2 PLS2 PLS2 XL1K XL1K IMAGING XL1K AUXILIARY DUMMY TOOL AUXILIARY AUXILIARY AUXILIARY CALIPER CALIPER CALIPER CALIPER CALIPER CALIPER PLS2 XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K XL1K
    • Contractor Tool Mnemonics 305 Baker Atlas Tool Mnemonics Tool 3VSP 4CAL AC AGN AP BAL BPS BRDL CAL CBIL CBL CCL CDB CDL CENT CH CHL CHTS CMI CN CO DAC DAL DCEN DEL2 DEL4 DEN DFS DHPA DIFL DIP DLL DMAG DPIL DSL DVRT FCON FDDP FDN FMCS FMFI FMT Description 3-AXIS GEOPHONE 4 ARM CALIPER ACOUSTIC LOG AIRGUN AUTOMATIC DIFFERENTIAL PRESSURE VALVE BOND ATTENUATION LOG FMT BYPASS SUB CABLEHEAD WITH 85 FEET OF BRIDLE MATERIAL AND SP ELECTRODE CALIPER CIRCUMFERENTIAL BOREHOLE IMAGING LOG CEMENT BOND LOG CASING COLLAR LOCATOR DUMP BAILER COMPENSATED DENSITY LOG CENTRALIZER CABLEHEAD WITH SP ELECTRODE BUTTON CHLORINE LOGGING TOOL CABLEHEAD TENSION MEASUREMENT COMPACTION MONITORING INSTRUMENT COMPENSATED NEUTRON LOG CARBON/OXYGEN LOG DIGITAL ARRAY ACOUSTILOG LOG DIGITAL ACOUSTIC LOG DE-CENTRALIZER 200 MHz DIELECTRIC LOG 47 MHz DIELECTRIC LOG NON-COMPENSATED DENSITY LOG DISPLACEMENT FLUID SAMPLER DOWNHOLE POWER ADAPTOR DUAL INDUCTION FOCUS LOG DIPLOG DUAL LATEROLOG DIGITAL MAGNELOG DUAL PHASE INDUCTION LOG DIGITAL SPECTRALOG DIGITAL VERTILOG WELLBORE FLUID CONDUCTIVITY (SALINITY) LOG FLUID DENSITY FROM DIFFERENTIAL PRESSURE LOG FLUID DENSITY LOG FLOWMETER—CONTINUOUS SPINNER FLOWMETER—FOLDING IMPELLER FORMATION MULTI-TESTER
    • 306 Tool FPI FS GP GR GRC GRN HDIL HDIP HDLL HP HOIS HTD HYDL IEL ISSB JBSK KNJT LL3 M5M7 MAC MAC2 MAG MCFM MFC MFP ML MLL MRIL MSI MSL MST NEU NFL NIR NLL NO ORIT PDK PFC PHT PNHI POS PROX PRSM Well Logging and Formation Evaluation Description FREE POINT INDICATOR (PIPE RECOVERY) BOREHOLE FLUID SAMPLER GRAVEL PACK (SHAKER) GAMMA RAY LOG PRESSURE (GRC STRAIN GAUGE) GAMMA RAY/NEUTRON COMBINATION HIGH DEFINITION INDUCTION LOG 6-ARM (HEX) DIPLOG HIGH DEFINITION LATEROLOG PRESSURE (HP QUARTZ GAUGE) SURFACE SYSTEM PANEL (HOIST/DEPTH) HIGH TEMPERATURE DENSITY LOG HYDROLOG INDUCTION ELECTROLOG MASS-ISOLATION JOINT JUNK BASKET KNUCKLE JOINT LATERALOG (3 ELECTRODE) CROSSOVER POINT FOR M5 TO M7 OR M7 TO M5 MULTIPLE ARRAY ACOUSTIC LOG MULTIPLE ARRAY ACOUSTIC LOG MAGNELOG (ANALOG) MULTI-CAPACITANCE FLOW METER MULTI-FINGER CALIPER CONTINUOUS MAGNETIC FREE POINT SENSOR MINILOG MICRLATEROLOG MAGNETIC RESONANCE IMAGING LOG MULTIPARAMETER SPECROSCOPY INSTRUMENT MICRO SPHERICAL LATEROLOG WITH CALIPER HEAVY-WALLED DRILL PIPE SEVERING WITH SYNCHRONIZED DETONATION SINGLE DETECTOR NEUTRON TRACER/FLO-LOG NEAR INFARED INSTRUMENT NEUTRON LIFETIME LOG NUCLEAR ORIENTATOR ORIENTATION LOG PULSE/DECAY PULSED NEUTRON LOG PERFORATING FORMATION CORRELATION LOG PHOTON LOG PULSED NEUTRON HOLDUP INDICATOR POWERED ORIENTAION SWIVEL PROXIMITY LOG PRISM LOG (MULTIPLE ISOTOPE TRACING)
    • Contractor Tool Mnemonics Tool RCI RCOR ROLR RPL RPM SBT SG SGEO SJ SK SL SLAP SLKP SNKB SON SPCR SPDK SPSB SRPL SSP STAR STFP SUB SUB SWC SWN SWVL TBFS TBRT TCAL TCP TCR TEMP TILT TMFP TPFM TTRM VRT VSP VTLN WCAL WESB WHI Description RESERVOIR CHARACTERIZATION INSTRUMENT ROTARY SIDEWALL CORING TOOL ROLLER BODY PANEX PRESSURE GAUGE MULTI-PURPOSE SMALL DIAMETER PULSED NEUTRON LOGGING INSTRUMENT SEGMENTED BOND TOOL STRAIN GAUGE SURFACE GEOPHONE SILVER JET PERFORATOR SELECTKONE PERFORATOR SPECTRALOG SIMULTANEOUS LOGGING AND PERFORATING SLIMKONE PERFORATOR SINKER BAR SONAN (NOISE LOG) SPACER BAR SPECTRAL PULSE/DECAY PULSED NEUTRON LOG SP (SPONTANEOUS POTENTIAL) SUB SURFACE RECORDED PRESSURE LOG SEMI-SELECT PERFORATOR SIMULTANEOUS ACOUSTIC-RESISTIVITY IMAGING LOG PIPE FREE POINT INDICATOR GYRO DATA INTERCONNECT AC/DC SWITCHING CIRCUIT TO RUN PHOTON & GRAVEL PACK IN COMBO SIDEWALL CORGUN SIDEWALL NEUTRON SWIVEL THROUGH TUBING FLUID SAMPLER THIN-BED RESISTIVITY LOG THROUGH TUBING CALIPER TUBING-CONVEYED PERFORATING THROUGH CASING RESISTIVITY TEMPERATURE LOG TRANSVERSE INDUCTION LOG MAGNA-TECTOR FREE POINT INDICATOR THERMAL PULSE FLOWMETER TEMPERATURE, TENSION AND MUD RESISTIVITY SUB VERTILOG VERTICAL SEISMIC PROFILE VERTILINE BOWSPRING 6-ARM CALIPER SIDE ENTRY SUB FOR PIPE CONVEYED LOGGING WATER HOLD-UP INDICATOR 307
    • 308 Tool WTS WTSP WTSS XMAC ZDL Well Logging and Formation Evaluation Description WIRELINE TRANSMISSION SYSTEM DOWNHOLE TELEMETRY REPEATER WIRELINE TRANSMISSION SYSTEM SURFACE PANEL WIRELINE TRANSMISSION SYSTEM SWITCHING SUB CROSS MULTIPOLE ARRAY ACOUSTILOG Z-DENSITY LOG
    • BIBLIOGRAPHY Basic Evaluation Techniques Archie, G. E. “The Electrical Resistivity Log as an Aid in Determining Some Reservoir Characteristics.” Petroleum Technology 5 (January 1942). Juhasz, I. “Porosity Systems and Petrophysical Models Used in Formation Evaluation.” SPWLA London Chapter Porosity Seminar, 26 April 1988. Core Analysis Anderson, W. G. “Wettability Literature Survey Part 1: Rock/Oil/Brine Interactions and the Effects of Core Handling on Wettability.” Journal of Petroleum Technology 38 (October 1986): 1125–1144. Rathmell, J. J., Tibbitts, G. A., Gremley, R. B., Warner, H. R., & White, E. K. “Development of a Method for Partially Uninvaded Coring in High Permeability Sandstones.” Paper 20413 presented at SPE Annual Technical Conference, Formation Evaluation, New Orleans, June 1990. Rathmell, J. J., Gremley, R. B., & Tibbitts, G. A. “Field Applications of Low Invasion Coring.” Paper 27045 presented at SPE Annual Technical Conference, Houston, 3–5 October 1993. Skopec, R. A. “Proper Coring and Wellsite Core Handling Procedures: The First Step Towards Reliable Core Analysis.” Journal of Petroleum Technology 46 (April 1994). Fluid Replacement Modeling Gassmann, F. “Uber die elastizitat poroser medien [Elasticity of Porous Media].” Vierteljahrsschrift der Naturforschenden Gesselschaft 96, 1951: 1–23. 309
    • 310 Bibliography Logging Tool Types Wahl, J. S., Tittman, J., & Johnstone, C. W. “The Dual Spacing Formation Density Log.” Journal of Petroleum Technology, December 1964. Homing In de Lange, J. I., & Darling, T. J. Improved Detectability of Blowing Wells. Paper 17255 presented at SPE/IADC, March 1988. Jones, D. L., Hoehn, G. L., & Kuckes, A. F. Improved Magnetic Model for Determination of Range and Direction of a Blow-Out Well. SPE paper 14388, September 1985. Kuckes, A., et al. An Electromagnetic Survey Method for Directionally Drilling a Relief Well into a Blown-Out Oil or Gas Well. SPE paper 10946, March 1981. Mitchell, F. R., Robinson, J. D., et al. “Using Resistivity Measurements to Determine Distance Between Wells.” Journal of Petroleum Technology, June 1972: 723. Multimineral/Statistical Models Mayer, C., & Sibbert, A. “GLOBAL, A New Approach to Computer Processed Log Interpretation.” Paper 9341 presented at SPE Annual Tech Conference, 1980. NMR Logging Coates, G. R., Gardner, J. S., & Miller, D. L. “Applying Pulsed Echo NMR to Shaly Sand Formation Evaluation.” Paper presented at SPWLA Convention, Oklahoma, June 1994. Slijkerman, W. F., & Hofman, J. P. “Determination of Surface Relaxivity from NMR Diffusion Measurements.” Magnetic Resonance Imaging 16 (June–July 1998): 541–544. Production Geology Issues Shell Internationale Petroleum Maatschappij BV (SIPM) Training Division. Course notes from Shell “Phase 2” Production Geology course, 1986.
    • Bibliography 311 Reservoir Engineering Issues Dake, L. P. Fundamentals of Reservoir Engineering. Amsterdam: Elsevier, 1978. Katz, D. L., et al. Handbook of Natural Gas Engineering. New York: McGrawHill, 1959. Marsal, D., for SIPM. Course notes from Shell “Phase 1” Reservoir Engineering course, 1982. Rock Mechanics Issues Biot, M. “Theory of Elasticity and Consolidation for a Porous Anisotropic Solid.” Journal of Applied Physics 26 (1955): 182–185. Brown, R. J. S., & Kerringa, J. “On the Dependence of the Elastic Properties of a Porous Rock on the Compressibility of the Pore Fluid.” Geophysics 40 (1975): 608–616. Saturation/Height Functions Leverett, M. C. “Capillary Behaviour in Porous Solids.” Transactions of the American Institute of Mining and Metallurgical Engineers 142 (1941): 152–169. Shaly Sand Analysis Clavier, C., Coates, G., & Dumanoir, J. L. “The Theoretical and Experimental Basis for the ‘Dual Water’ Model for the Interpretation of Shaly Sands.” Society of Petroleum Engineers Journal, April 1984: 153–167. Juhasz, I. The Central Role of Qv and Formation Water Salinity in the Evaluation of Shaly Formations. Paper presented at the Society of Professional Well Log Analysts 19th Annual Symposium, June 1979. ——. “Normalised Qv: The Key to Shaly Sand Evaluation Using the WaxmanSmits Equation in the Absence of Core Data.” Paper presented at SPWLA Symposium, 1981. Waxman, M. H., & Smits, L. J. M. “Electrical Conductivities in Oil-Bearing Shaly Sands.” Society of Petroleum Engineers Journal, June 1968: 107–119. Trans AIME 243. Waxman, M. H., & Thomas, E. C. “Electrical Conductivities in Oil-Bearing Shaly Sands: I. The Relation Between Hydrocarbon Saturation and Resistivity Index. II. The Temperature Coefficient of Electrical Conductivity.” Society of Petroleum Engineers Journal 14 (1974): 213–225.
    • 312 Bibliography Worthington, P. F. “The Evolution of Shaly Sand Concepts in Reservoir Evaluation.” The Log Analyst, January–February 1985: 23. Thermal Decay Neutron Interpretation Clavier, C., Hoyle, W. R., & Meunier, D. “Quantitative Interpretation of TDT Logs, Parts I and II.” Journal of Petroleum Technology, June 1971. Thin Beds Moran, J. H., & Gianzero, S. “Effects of Formation Anisotropy on Resistivity Logging Measurements.” Geophysics 44 (1978): 1266–1286. Thomas, E. C., & Steiber, S. J. “The Distribution of Shales in Sandstones and Its Effect Upon Porosity.” Transactions of the 16th Annual SPWLA Logging Symposium, 1975, paper T. Well Deviation and Geosteering Wolff, C. J. M., & de Wardt, J. P. “Borehole Position Uncertainty: Analysis of Measuring Methods and Derivation of Systematic Error Model.” Journal of Petroleum Technology, December 1981: 2339–2350.
    • ABOUT THE AUTHOR Toby Darling was educated at The Pilgrim’s School and Winchester College before gaining a scholarship to Wadham College, Oxford, where he graduated with a degree in physics in 1982. He immediately joined Shell International Petroleum Maatschappij (SIPM). After initial training in The Hague, he was posted to Thai Shell from 1983 to 1984, working as a wellsite petroleum engineer. In 1984 he was posted to SIPM in The Hague, working on secondment to the Exploration seismic group; in 1986 he transferred to the Shell Research Laboratories (KSEPL), where he worked on a number of novel logging techniques, including the development of LWD tools. In 1989 he was seconded to Petronas Carigali Sdn Bhd in Miri, Malaysia, where he worked as an operational petrophysicist in the Baram Delta region. In 1992, he moved to Shell Expro in Aberdeen, working as a petrophysicist and team leader for a number of North Sea fields. In 1997 he transferred to Shell Expro’s exploration department in London, where he worked until 1999 before leaving Shell to form his own consultancy. Since 1999 he has worked as a consultant to a number of oil companies, most recently on contract to BAPETCO in Cairo. He is married with two sons. 313
    • ACKNOWLEDGMENTS I have been fortunate during most of my career to have worked for a major oil company (Shell) which has been at the forefront of petrophysics. I am therefore grateful to Shell for providing the training and environment that has made this book possible. I would like to thank BAPETCO and Shell Egypt for permission to use some of their log data in the exercises and for field examples of logs. In particular I would like to thank Erik Jan Uleman for his help in reviewing the inital draft of the book and Mahmoud M. Anwar for his suggestions concerning horizontal well permeability. I would like to thank Schlumberger, Halliburton, and Baker Atlas for permission to include lists of their tool mnemonics. 314
    • INDEX A Abnormal pressures, 153–154 Accelerometer, 174, 176 Acetone, 23 Acidization, 27, 74 Acoustic impedance, 103–108 modeling, 110–113 Air, density of, 158 Allogenic clays, 148 Amplitude versus offset (AVO), 112 Angle, contact, 60, 61 Anhydrite, 73, 75 Anomalies, magnetic, 173 Anticline, 153 Aquathermal pressures, 154 Arbitration, pendulum, 129 Archie approach, 132 equation, 36, 37, 67, 71 model, 96 Area-depth graph, 144 Authigenic clays, 149 Average arithmetic, 54 geometric, 55 harmonic, 55 Averages permeability, 54 petrophysical, 40 AVO—see Amplitude versus offset Axial plane, 153 Azimuth, 174, 194 B B factor, 67 back-up, 11 Bandwidth, LWD, 199 Barite, 52 Barrels of oil equivalent, 126 Battery life, 4 Beds, thin, 86, 87 Bg (gas formation volume factor), 161 Bins, T2, 80 Biot-Savart’s law, 187 Bitumen, 22 Blocking, gas, 56 Blow-outs, 171 Bo (oil formation volume factor), 3, 161 BOE (barrels of oil equivalent), 126 Bond, cement, 8 Borehole corrections, 101 Borehole position uncertainty, 196 Bound water mode, 83 Boundaries, 125 Breakthrough, water, 26 Bridle, 187 Bubble point, 159 315
    • 316 Index Build-up analysis, 169–169 Bulk modulus, 109 Butterworth filter, 105 BVI, 81 C C constant, Waxman-Smits equation, 69 Calcisands, 150 Calcite, 22 Calibration, 10 Caliper, 6, 30, 74 CapEx, 119 Capillary effects, 203 Capillary pressure, 19 air/mercury, 59 core, 60 Carbonate reservoirs, 149–151 Carbonate, particle type, 22 Carbonates, 73 Cased hole logging, 7 Casing, 172 Casing, collar locator, 8 CAT scanning, 18 Cation exchange capacity (CEC), 36, 67, 68, 149 CBW, 81 CEC—see Cation exchange capacity Cement bond, 26 log, 8 Cement, Gilsonite, 22 Cementation, 150 exponent, 18 Centipoise, 46 Centrifuge, 19 Centrifuging, 17 Chalk, 73 Chambers, formation pressure, 7 Charge cable splice, 10 data, 10 depth, 9 engineer, 9 lost-in-hole, 10 processing, 10 station, 9 survey, 9 transmission, 10 Charges, perforating, 25 Checkshot, 7 Chlorite, 68, 149 Chloroform, 23 Chlorothene, 23 Clastic reservoirs, 30, 147–149 Clay, 75 minerals, 149 typical properties, 68 Clay-bound water, 17, 19 Cleaning, 20 Closest Approach, point of, 183 Collars, casing, 24 locator, 8 Color, sample, 22 Compaction, 115–118, 150 disequilibrium, 154 Compressibility composite, 162 porespace, 162 Compressional velocity, 109 Compton Scattering, 5, 35 Condensate, 23, 159 Conductivity, excess, 68 Contact angle, 60, 61 reservoir conditions, 63 Contact, horizontal wells drilled above, 203–205 Contouring methods, 142–143 Contours, 140 Contracts, 9 Core cleaning, 20 drying, 20 freezing, 16 limitations, 20 plugging, 20
    • Index slabbing, 17 stress, 20 Core data, 50 Coring, 16, 139 conventional, 16 resin, 16 sidewall, 7 sponge, 16 Corrections, borehole, 101 Costs, abandonment, 119 CPMG excitation, 78 pulses, 80 Crest, 153 Crestal plane, 153 Critical point, 159 Crossplot density/neutron, 37 Log(J) vs Log (Swr), 64 Cusping, gas, 26 Cut, solvent, 23 Cut-and-thread, 15 Cutoff, 32, 51, 97, 130 permeability, 41 T2, 79 Vsh, 133 Cuttings description, 21 Cwa, 69 D Darcy’s Law, 163–166 Data charge, 10 Database, corporate, 49 datum, 47, 193 DC component, 104 Dean-Stark, 17 Decay, thermal, 8, 93–95 Decision tree, 121–124 Decisions, operational, 14 Declination, 175, 194 Deemed equity, 125 Deep marine, 148 317 Deepwater carbonates, 150 Deformation, 151–153 Deltaic, 148 Density, 5, 103–108, 163 fluid, 34, 35, 52 formation water, 60 gas, 130 hydrocarbon, 60 log, 31 matrix, 34, 35, 73 oil, 130 up/down, 197 water, 130 wireline, 6 Depletion, 65, 93, 117, 154, 160, 203 Depth, 193 determination, 195 subsea, 32 Depth charge, 9 Derrick floor, 193 Description, cuttings, 21 Desert, 147 Detection gas, 24 hydrocarbon, 22 Determination, equity, 125–135 Development well, 12 Deviation, well, 193–195 Dew point, 159 Diagenesis, 20 Diagenetic effects, 53 Diamagentic ions, 84 Diapir, 151 Differential spectrum method (DSM), 81 mode, 82 Differential sticking, 15 Differentiation, gas-oil, 34 Dimensionless time, 167 Dip, 17 apparent formation, 202 direction, 140 magnitude, 140
    • 318 Index sedimentary, 7 stratigraphic, 7 true formation, 202 Dipole, field due to, 181 position of, 183–184 Dipole sonic tool, 118 Disequilibrium, compaction, 154 Distribution acoustic impedance, 111 normal, 100 T2, 80 Dogleg, 8 Dolomite, 22, 73, 75 Dolomitization, 151 Drager tubes, 24 Drainage, 19, 61 Drainage area, 164 Drawdown, 161 Driller’s depth, 18 Drillpipe, 15 Drillstring, 172 Dry test, 44 Drying, 20 DSM—see Differential spectrum method Dual water model, 72 E Earth’s magnetic field, 171, 175 Eckert number, 39 EHC (net*porosity*hydrocarbon volume), 145–146 Elastic impedance modeling, 110–113 Electromagnetic homing-in, 185–191 principles, 186 DEMV (estimated additional money value), 121–123 Equations, deterministic, 75 Equity determination, 125–135 Equity, deemed, 125 Error analysis, 96–101 ESP (electric submersible pump), 27 Ethane, 158 Ether, 23 Euler’s constant, 165 Expert determination, 128 Expert guidance, 129 Exploration well, 12 Exponent, cementation, 18, 36, 70 Exponent, saturation, 19, 36, 70 Exponential integral, 165 F Facilities, 120 Failures, tool, 14 Faulting, 151–152 FFI (free fluid index), 81 Field development plan (FDP), 3, 11 Filtrate, mud, 43 resistivity, 13 FINDS, 196 Fishing, 11 Fixed Equity, 128 Flatlining, 29 Flow velocity, 163 Fluid contacts, 133 Fluid replacement modeling, 108–110 Fluids, NMR properties, 83 Fluoresecence, 33 natural, 22 Fluvial, 147 Flux, magnetic, 172 FOL (free oil level), 44 Folding, 152–153 Formation dip apparent, 202 true, 202 Formation factor, 53, 70 Formation pressure, 7 Formation water density, 60 Formation water resistivity, 36
    • 319 Index Fractures, 17, 73 Fracturing, 27 Free water level (FWL), 1, 42, 44, 45, 60, 64, 65, 126, 130, 134, 203 Freezing, 16 Function, membership, 86 Fuzzy logic, 85, 110–113 FWL—see Free water level G Gamma, 143 Gamma-ray (GR), 24, 30, 50, 74, 85 LWD-, 5 up/down, 197 wireline, 5 Gas blocking, 56 caps, 33 constant, 155 cusping, 26 detection, 21 factor, 130 formation volume factor (Bg), 161 initially in place (GIIP), 1, 41, 59, 73 oil contact (GOC), 2, 32, 34, 42, 44, 130, 134, 161 peaks, 33 water contact (GWC), 2, 44, 45 Gases behavior of, 155–158 behavior of oil/wet, 159–162 properties of, 158 Gas-oil differentiation, 34 Gassmann equations, 108–110, 111 Gauge, strain, 47 GBV—see Gross bulk volume Generic saturation/height functions, 63, 65 Geologist, 12 Geophone, 7 Geosteering, 196, 197–203 up/down response, 201 GIIP—see Gas, initially in place Glacial, 148 GOC—see Gas, oil contact GR—see Gamma ray Gradients, anomalous, 44 Grain density, 18 Gravitational constant, 163 Gravity, 193 Grid north, 194 Gross bulk volume (GBV), 1, 126, 131, 143, 145 Gross Thickness, 2 Growth faults, 151–152 Guns, 25 GWC—see Gas, water contact Gyro survey, 47 tools, 196 Gyromagnetic ratio, 77 H H2S, 21 Hardness, sample, 22 HCPV—see Hydrocarbon pore volume Height above free water level, 60 Highside, 173 HIIP—see Hydrocarbons initially in place Hinge line, 153 Histogram, 85 grain density, 51 History-matching, 57 Holding-up, 15 Homing-in electromagnetic, 185–191 techniques, 171–191 magnetostatic, 171–185 Horner plot, 168–169 HSTF (angle between highside and toolface), 177
    • 320 Index Humble constant, 37 HWC—see Hydrocarbon/water contact Hydrocarbon density, 60 Hydrocarbon pore volume (HCPV), 1, 126, 131 Hydrocarbon/water contact (HWC), 2, 134 Hydrocarbons initially in place (HIIP), 137–138, 143 Hydrogen, 77 Hydrogen index, 8 Hydrostatic conditions, 116 I Ideal gas law, 155 Illite, 68, 149 Imaging, 6, 74 resistivity, 13 Impedance, acoustic, 103–108 modeling, 110–113 Impedance, elastic modeling, 110–113 Inclination, 174, 193 Increments, sampling, 10 Index, hydrogen, 8 Index, resistivity, 53 Indonesia equation, 72 Induction, 6, 13 Information, value of, 119–124 In-hole charge, 9 Injection, continuous, 19 Interfacial tension, 60, 61 reservoir conditions, 63 Invaded zone, 76 Invasion, 4, 17, 32, 52, 64, 102 Inverse distance, 142 Investgation, depth, 84 Irreducible water saturation, 60 Isochore, 141 Isopach, 141 Isostatic conditions, 116 J J function, 59, 60, 61, 88, 130, 204 core derived, 64 fitting, 62 log derived, 64 Joint Operating Agreement, 127 Juhasz method, 71 K Kaolinite, 68, 149 Kelly bushing, 193 Kerosene, 19 Key seating, 15 kh, 56 Kriging, 142 L Laminae, 88 Larmor frequency, 77 Laterolog, 6, 13 Leaching, 151 Leak-off test, 117 Leverett J function, 59 Limb, 153 Limitations, NMR, 84 Logger’s depth, 18 Logging while drilling (LWD), 3, 84, 199 cased hole, 7 mud, 21 pipe-conveyed, 8 production, 8 Logic, Fuzzy, 85 Lost-in-hole charge, 10 LWD—see Logging, while drilling M m, 130 Magnetic north, 194
    • 321 Index Magnetometer, 171 Magnetostatic homing-in, 171–185 Management negotiation, 128 Mapping, 196 Maps, 141 Material balance, 162–163 Matrix density, 34 Matrix Poisson ratio, 109 Matrix shear modulus, 109 Mean, 99 Mean sea level, 193 Measurement while drilling (MWD), 172 Mechanics, rock, 115–118 Membership function, 86 Memory, LWD, 4 Mercury, 19 Meridian, 194 Methane, 158 MgCl, 39 Microresistivity, 6, 73 Minerals, 17 clay, 149 Minimum phase, 105 Mobility, 45 Model dynamic, 2, 59 static, 2, 59 Modeling acoustic impedance, 110–113 elastic impedance, 110–113 fluid replacement, 108–110 Molecular self-diffusion, 78 Monopole, 172–185 field due to, 178–180 Monte-Carlo analysis, 96–101 Montmorillonite, 68, 149 Mud filtrate resistivity, 13 water-based, 6 Mudlog, 21 Mudlogging, 21 Multimineral models, 74 MWD—see Measurement while drilling N n, 130 NaCl concentration, 38 Net effective vertical stress, 115 Net pore volume (NPV), 1, 126, 131 Net present value (NPV), 3, 119 Net sand, 49 Net thickness, 2 Net/gross, 97, 132, 139 Neural networks, 87 Neutron, wireline, 6 Neutron, LWD, 5 Neutron, thermal decay, 93–95 Nitrogen, 158 NMR—see Nuclear magnetic resonance Nonmagnetic collars, 196 Nonskeletal particles, 22 Non-wetting phase, 19 Normal distribution, 100 Normal faults, 151–152 Normalized Qv method, 71 North pole, 172 North, grid, 194 North, magnetic, 194 North, true, 195 NPV net pore volume, 1, 126, 131 net present value, 3, 119 Nuclear magnetic resonance (NMR), 7, 54, 76, 199 O OBM—see Oil-based mud Odor, 23 Oil-based mud (OBM), 6, 13, 30, 54, 74, 80 Oil formation volume factor (Bo), 161
    • 322 Index Oil volume factor, 3 Oil water contact (OWC), 2, 32, 45, 143, 160, 203 from core, 17 Oil, dead, 22 OpEx, 119 Osmosis, 154 Overbalance, 32 Overburden, 18, 115 Overpressures, 115 Overshot, 15 OWC—see Oil water contact P P15, 2 P50, 2 P85, 2 Packer, 43 Paramagnetic ions, 84 Pay, 41 Pay thickness, 2 Pellets, 148 Pendulum arbitration, 129 Perforations, 26 Permeability, 1, 45, 47, 54, 60, 139, 163 carbonate, 73 core, 17 horizontal wells, 55 relative, 26 sample, 22 test, 56 Phase changes, 154 Phase, minimum, 105 Phasing, guns, 25 Photoelectric effect, 6 PI, 166 Pickett plot, 38 Pinch-out, 107 pip, 8 Pi-pulse, 77 Planimeter, 144 Plugging, 20 Plugs core, 17 horizontal, 17 vertical, 18 Point of closest approach, 183 Poisson ratio, 109, 116 Polarization, longitudinal, 80 Pole north, 172 south, 172 Polynomial fit, 142 Population, 99 Poroelastic constant, 116 Poroperm equation, 138 Poroperm relationship, 47, 50, 51, 62 Porosimeter, 17 Porosity, 1, 36, 51, 60, 133 calculation, 32, 34 carbonate, 73 clay, 35 core, 17, 51 core calibration, 52 effective, 36 neutron, 6 sample, 22 total, 36 Potassium, 6 Potential, spontaneous, 6 Pressure solution, 150 Pressure, capillary, 19, 45 Pressure, formation, 7, 44 Pressure, tubing head, 25 Pressures, abnormal, 153–154 Pretest, 7, 47 Probability, relative, 86 Probe, 7 Procedures, equity determination, 127 Processing charge, 10 Production geology, 137–154 Production logging, 8 Production cumulative, 3 tail-end, 11
    • 323 Index Productivity, 166 Productivity Index, 166 Productivity, horizontal wells, 205 Profiling, vertical seismic, 7 Propane, 158 P-T diagram, 161 PVT (pressure-volume-temperature), 43, 160 sampling, 161–162 Pyrite, 22, 75 Q Quality Control, 29 Quartz, 75, 148 Quicklook Interpretation, 29 Qv, 40, 68 R Radial flow, 164 Radioactive pip tag, 25 Randomness, 97 Rate of penetration (ROP), 4, 5, 21, 197 Ratio, gyromagnetic, 77 Real-time, 4 Recombination samples, 160 Recompletion, 27 Recovery factor, 3 Recovery, ultimate, 3 Reefs, 150 Reflectivity, 104 Regularization, 80 Relaxation, transverse, 77 Relaxivity, surface, 78 Reliability, 121–124 Rental, base, 9 Rental, tool, 9 Repeat section, 11 Reperforation, 27 Reserves, 2, 126 developed, 3 possible, 3 probable, 3 proven, 3 remaining, 3 Reservoir depleted, 34 identification, 30 Reservoir engineering, 155–170 Resistivity deep, 30 formation water, 36 LWD, 5 shallow, 30 true, 38 wireline, 6 Resistivity index, 53 Results, presentation, 40 Retrograde condensate, 159 Reverse faults, 151–152 Rmf (mud filtrate resistivity), 13 Rock mechanics, 115–118 Rollover anticlines, 151 ROP—see Rate of penetration Roundness, sample, 22 Rs (solution gas/oil ratio), 161 Rw, 54, 130 S Safety, 14, 21 well, 196 Salinity, formation, 43, 53 Salt domes, 151 Sample dry, 22 segregated, 43 wet, 22 Sampling formation, 7 pressure, 42 sidewall, 7 Sampling theory, 99 Sand, net, 49
    • 324 Sands clean, 31 laminated, 32 water-bearing, 33 Sandstone, 35 Saturation, 133 Archie, 53 hydrocarbon, 1 hydrocarbon, 37 hydrocarbon, 81 irreducible, 45 irreducible water, 60 water (Sw), 1, 36 Saturation/height, 54 analysis, 59 Saturation exponent, 19 SCAL—see Special core analysis Scalar product, 176 Scales log, 11 presentation, 30 Scanning, CAT, 18 Scattering, Compton, 5, 35 Scope for recovery, 3 Seal failure, 43 Sealed bids, 128 Sedimatnation, 150 SEG (Society of Exploration Geophysicists), 107 Seismic integration with, 103–108 vertical profiling, 7 Seismograms, synthetic, 103–108 Self-diffusion, molecular, 78 Semi-steady state, 164, 167 Semivariogram, 142 Sensor plane, 173 Shale conductive, 88 dispersed, 50 dispersed, 89–92 laminated, 89–92 structural, 89–92 Index Shale volume, 32, 50 Shales, 31 Shallow marine carbonates, 150 Shaly sand, 67 Shear modulus, 109 Shear velocity, 109 Shelf, 15 Shifted spectrum method (SSM), 81, 82 Shoreline, 148 Shoulder bed effects, 64, 101 SHPOR, 41 Side-entry sub, 8 Siderite, 22, 75 Sidewall coring, 7 Sidewall sampling, 7 Sigma, 93–95 Simandoux equation, 72 Simulator, 63 Skin, 56, 166 Slabbing, 17 Smearing, 175 SO2, 21 Solution gas/oil ratio (Rs), 161 Solution, pressure, 150 Solvent, 17 Solvent cut, 23 Sonic log, 30, 73 Sonic integrated, 104 LWD, 5 wireline, 6 Sorting, sample, 22 Sources, nuclear, 16, 85 South pole, 172 SP—see Spontaneous potential Special core analysis (SCAL), 18, 53, 60, 116 Spectroscopy, gamma-ray, 8 Spectroscopy, natural gamma-ray, 5 Spectrum, differential method, 81 mode, 82 Spectrum, T2, 80
    • 325 Index Sphericity, sample, 22 Spin, 77 Spontaneous potential (SP), 6, 13, 39 SSM—see Shifted spectrum method Staining, visible, 23 Standard deviation, 99 Standing & Katz, 156–157 Station charge, 9 Stations, survey, 195 Steady state, 164 Sticking, differential, 15 Stock tank oil initially in place (STOIIP), 1, 41, 59, 73, 92, 120, 145 Strain gauge, 47 Strategy, logging, 11 Stress, vertical, 115 Strike direction, 140 Strike line, 140 Stuck tools, 14 Supercharging, 43 Surfactants, 80 Survey, gyro, 47 Survey charge, 9 Survey tools, gyro, 196 Survey well, 183 Surveying, 195–197 Surveying, legal factors, 197 Sw (water saturation), 1 Swamps, 148 SWPOR, 99 Syncline, 153 Synthetic seismograms, 103–108 T T1, 7, 77 T2, 7, 78, 79 Target well, 183 TDA—see Time domain analysis TDT—see Thermal decay time Telemetry, 200 Temperature ambient, 20 bottomhole, 39 maximum, 13 Tension, 29 Tension, interfacial, 60, 61 Testing, 24 Testing, well, 166–170 Texture, sample, 22 Thermal decay neutron, 93–95 Thermal decay time (TDT), 124 Thermal decay tool, 8 Thin beds, 86, 87 Thomas-Steiber plot, 89–92 Thorium, 6 Time domain analysis (TDA), 78 Time, dimensionless, 167 Time, wait, 79 Time-lapse, neutron, 94 Time vs. depth (TZ) graph, 104 Tool failure, 4 Toolface, 173 Tools, back-up, 11 Tools, stuck, 14 Toolstrings, 13 Total porosity mode, 83 Tramline, 32 Transmission charge, 10 Tree, decision, 121–124 Triangulation, 142, 189 True north, 195 True vertical depth (TVD), 47, 180, 193, 203 Tw, 79 Two way time (TWT), 104 Type, sample, 22 TZ (time vs. depth) graph, 104 TZ relationship, 108 U Ultraviolet (UV) light, 17, 22, 23, 50 Uncertainty, 11, 96–101
    • 326 Index Uncertainty, borehole position, 196 Uncertainty, TVD, 203 Unitization, 125 Up/down GR, 197 Uranium, 6 V Value of information (VOI), 119–124, 139 van der Waals equation, 156 Vapor pressure line, 155 Varwiggle format, 105 Velocity compressional, 6, 34, 109 shear, 6, 34, 109 Vertical seismic profile (VSP), 104 Vertical seismic profiling, 7 Virgin zone, 76 Viscosity, 163 diesel, 46 gas, 46 oil, 46, 85 water, 46 VOI—see Value of information Vp, 103–108 Vsh, 32, 50, 68, 133 VSP—see Vertical seismic profile Vugs, 73 W Wait time, 79 Washouts, 36 Water, clay-bound, 79 Wavelet, 105 Waxman-smits equation, 67, 71 WBM, 13, 30, 80 Weakpoint, 15 Weight on bit (WOB), 198 Well development, 12 exploration, 12 Well deviation, 193–195 Well shoot test (WST), 7, 104 Well testing, 166–170 Wet-connect, 8 Whitening, 105 WOB—see Weight on bit Wrench fault, 152 WST—see Well shoot test Z Zonation, 138